Videos uploaded to YouTube that have ads enabled are explicitly not given away for free.
Maybe I loaded the link to the ad but never downloaded it; the site didn't require this. Maybe I downloaded the ad but my user agent chose not to display it. Maybe it did get displayed, and my eyes chose not to look at it. It's the same thing either way: I, the user, did not consume that content. All of this content is offered for free on request. There is no moral obligation to consume one piece of content just because it is delivered alongside another.
You're doing the digital equivalent of saying "the pumpkins are right there for me to take, they were delivered outside for me to pick up and place in my car, for free. With no exchange of currency. I can just walk by the store and grab one and walk away".
In other words, you're ignoring both the legal contract (the terms of service) and the social contract (if everyone just grabs a pumpkin and walks away its obviously not going to be profitable and the company will stop offering it, we can only have the social 'nice thing' if everyone plays nice).
Just because its physically possible to do a thing doesn't mean the company consents to you doing it, or you deserve to do it.
You can easily litter. There's nothing stopping you. The ground is right there. You shouldn't though and you're rightfully called an asshole if you do.
If you applied your mentality everywhere, you would say "if you don't want me to litter in this National Forest, there ought not to be any businesses that sell disposable products within a 500 mile radius. It's not my fault you set this up for me to do so."
When reading a magazine, do I need to start from the first page and spend a minimum number of seconds staring at the multiple full-page ads before flipping to an article I might actually be interested in?
Do we need to make VHS tapes and DVRs illegal, if they allow the user to record only the parts of the program they're interested in, while skipping over the commercial breaks?
The pumpkins argument is a strawman: with physical goods, theft and loss of value has occurred. The legal owner of the unpaid pumpkins (the store) is now unable to sell them to a paying customer. When I choose to watch one video, but not another, I have not somehow taken value away from anyone. The original copy of the data is still on the server, ready to be delivered to the next viewer.
The littering argument is a strawman: when I choose to watch one video, but not another, I have not committed an act of vandalism. No person other than me can even be aware that this has transpired, unless I tell someone else about it. What I choose to focus my attention on affects me, and me alone. Littering affects others, making it a poor comparison.
Of course the endgame - which we already see - is embedded video ads and product placement. "Hey youtube I'm going to provide the 10 second answer you were searching for, but first let me tell you how to lose money fast with crapcoin..." I was going to say that music doesn't have that yet but I'm not sure that is the case.