Not strictly ethical I guess but ¯\_(ツ)_/¯
But that moral argument is not the issue here.
Subscription services distill the marketplace for content that you can pay for. They make diverse media production prohibitively expensive, because customers can't afford to pay for a diverse collection of media when that collection is distributed across many subscriptions.
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?