I can see the following broad (and inter-related) issues here:
1. Work timings and its relation to nature of work:
The way I see it is that essentially they forced everyone
to work for an extra hour, because none of their other
duties where reduced.
What I meant is that when most of the participants in
the meeting joined the company, we joined with the idea
of a standard 9-6 job.
I see value in having them at a specific time. I see value
in having them early in the morning.
The 60-70 hours a week are not mandatory, but it's what's
needed to get things done. It just sounds unfair.
2. The business domain that the company is in, and how important is typical "tech culture" within the company.
It's a highly operations-driven company, so we need a lot
of communication, both us team leaders with our teams, and
between team leaders.
3. Your position in the company:
Now, I hold an important position in the company and I'm
very close to the founders [..]
[..] plus a mandatory leadership meeting on Saturdays
(11am-3pm), plus whatever bug arises at whatever time.
4. Some history behind the "draconian" decision, eg. 8am/9am decision, financial penalty for coming late, etc.
First, let's tackle the strict start time.
I have worked in a company with strict work timings. There was a similar meeting to kick-off the day and attendance was recorded. If you were late, even by a minute, a certain fixed amount was deducted from your pay (very nominal, btw). People with 100% attendance in a month, used to a get a lunch treat sponsored by the company. So, there was a stick, as well as a carrot here. I, and my friends, used to be barely on time, and late quite a few times. We used to bitch about this, but the truth is (and I realise this in hindsight), even if the meeting were to be shifted to 2 hours later, we would still be in the same position. You see, we were young & reckless during those days, and didn't sleep on time (parting, watching movies, etc). Only the "fear" of being late to work made us sleep on a reasonable time. If the meeting were to be shifted, our partying would go on till later in the night. We would still wake up just in time, skip breakfast, not take a shower, and rush to office!
(Btw, this is the reason I wanted to know about the age/work-ex profile in your company).
Further, I know of a close friend who is working in one of the largest electronic manufacturing brands in the world. Even though he works in the HQ, and not the factory floor, they have a strict start time (recorded by a biometric attendance system). If you are a minute late, it's recorded as a half working day, which can be regularised (i.e. converted back to a full working day), by putting a formal request to your manager + HR. Needless to say, the friction and embarrassment in putting repeated requests for attendance regularisation keeps people in check.
Given your closeness to the founders, you could suggest this, instead of deducting pay.
However, strict start times are just one side of the equation. What about getting off of work? Are most people working in shifts? (You mentioned, the "9-6" environemnt) Do most people (not you), get up and leave as soon as their shift ends, irrespective of whether their work has actually finished or not? This could be the reason for the founder getting irked and clamping down on strict start times.
Also, another thing to think about is, given the company's context, are strict start/end times required? Or is the founder trying to exert unnecessary control over employees? Your reading of the situation will change based on truthful analysis of this point. And it doesn't need to be a bitching session with other members in the team. Talk to your founder directly about this and see if he has a meaningful perspective to share.
Coming to your personal case, it seems that you are close to the founders and one some "leadership training track". Is the founder personally investing time in your growth? Is the Saturday meeting useful for you? Apart from the work start/end time, do you otherwise have autonomy and power in your work? If yes, then why do you resent the extra time you need to put? Are you growing faster (personally, professionally, and financially) by putting in 10-15 extra hours per week? If yes, then that is the cost of growing faster in life. You are compressing the calendar time to achieve the same amount of growth.
Again, this is not a universal truth. You, and you alone, can decide whether this tradeoff is acceptable to you. (Not other people who have heard only one side of the story and are advising you to "run away from that toxic place").
On a closing note, how old are the founders? Is this their first venture as top-level leaders? Is this the first time that they are managing medium / large-sized teams? Similar to how you may be learning tech as you go along, they may be learning people management as they go along. As a founder, sometimes you have a idea and not the necessary experience / skill-set to achieve that idea. Try talking to your founder about what outcome s/he is expecting by enforcing these rules (I'm sure there is more stuff apart from the 8am meeting and 50% pay cut). In your Satuday leadership meeting, try coming up with better workplace policies to achieve similar results. Unless the founders are genuine psychopaths they should be willing to give alternative methods a try.
Finally, every company has some shit that you have to deal with, and it has some "sugar" that you enjoy. Be sure you're striking the right balance between shit and sugar.