I’d file that under pirating. I don’t judge folks for pirating, but I think it’s a little rich to be like “why does anyone pay for things they could steal”.
What counts as “obligation” to you? You can also generally walk out of a restaurant without paying.
What I consider unethical is shoving them into my face in every way possible (frequently in ways to trick the user into clicking). I'd appreciate it more if ads were more like footnotes (with details on a separate page) that I'd visit and review if I wanted to.
Someone paid a fortune to put ads on busses to force me to see them. If I look away, am I somehow stealing from them?