It’s quite simply theft of resources.
I'm not sure I agree with the advertising is theft analogy, but this is a bad counter example.
A website is easily capable of requiring users to accept an actual agreement to see the content, but simply adding it to HTML isn’t a contract any more than listing it in some TOS somewhere is.
commercial radio
Many places in the world still are subject to metered data.
Imagine you ordered a book through the mail, knowing the book vendor offered it for free, but not knowing how, only after the fact finding out Toyota paid them to give away promotional pickup trucks, except you had to pay the shipping expense for a truck to go through the mail.
Is that theft? How is bloating the web with unrelated content a user has to pay for without knowing in advance they'll be getting it or even how much they'll be getting any different?
This is quite a bit different than all previous forms of advertising, which were directly embedded into the content media and added no cost to deliver. There was no way to order a newspaper and be delivered a video. Analog television streams were broadcast, with airwaves publicly allocated, not paid for by the consumer (you paid for the right to decode them).
Do I, though? If I've never been to the site before and don't know what's on it, my simply visiting the site for the first time isn't consent to display an auto-playing video.
I suspect, your point is that If I only steal bikes of people eating in my restaurant and I ask for permission if they are seated(1), then this is not stealing. Because of the permission. I would argue. It is an incredible.... bad deal for the one eating in my restaurant and it seems to be in bad faith.
It is even another story, if it is the only restaurant or food source in town. Then it goes well beyond stealing…
(1) And I certainly use all (bad/dark) tricks you can think of to get their permission.