I want a car that doesn't come with advertising built in. Every time I'm low on gas it starts dinging and flashing lights at me, and - can you believe this, people - if I refuse to buy any more, the car won't even start! And don't get me started on what happens every 10,000 miles... demanding I change the oil! I'm not a sucker, I won't fall for their advertising!
It's an ad because they are advertising the iCloud service to you. It's not just that they say "Your iCloud is running out of free space", it's that they say "Your iCloud is running out of free space, buy more space here!"
That's not an advertisement. That's a critical notification about a service you've chosen to use. It's not any different just because they offered a free trial.
No, I expect it not to try to sell me something at all. If I'm close to my capacity limit, it just means I need to clean some of the crud out.
It's not that they say "Your iCloud is running out of free space." or "Your iCloud is running out of free space, buy more space here!" it's that they say "sign up for iCloud and store your X!". The first two are status updates for a service that's already being used. The last is an ad.
It's an install option for OSX and you can trivially disable it as well. OSX runs just fine without it.
Also low space notifications for a service I am already using would be different than advertisements for a service I am not using at all.
The edge notifications look like a system alert and trigger constantly.