They're "allowed" to do whatever they want, and I'm allowed to think their complaining reeks of entitlement and selfishness.

So if a company X is offering something I would like but on different terms, you think it would be entitled and selfish of me to make it known publicly that I would prefer different terms?

The presumption is that they were using the product against the stipulated terms. Nobody is arguing against free speech and if you think Spotify are awful for serving bad ads you can say so.

But this article is specifically about people using a product against the stipulated terms. Criticizing bad ads is valid in general, but people expect topical discussions to stick to the context. Either clarify you are speaking from a different context or you will get misinterpreted.

I didn't use the term "allowed" to imply there is a free speech argument, but to imply that there is an unreasonable expectation on people's behavior. Expecting that customers should "just" stop using service without complaining about the fact that they are doing so is not a reasonable expectation. People should air their grievances in addition to voting with their feet.