I think your question is reasonable, but no, I do not think a company gets to promote a service as having no ads as part of the sell, and then put ads in by default.

They can do it legally if it's within the terms. Otherwise it's bait-and-switch.

I can't sign a contract with you for $10/month for no ads and then start showing you ads.

Because I have a better life to live than always monitoring which shitty company wants to scam me out of my attention and money next.