Stealing is taking property with the intention of permanently depriving the rightful owner of said property.
One cannot steal data by not watching ads because you can't deprive the owner of said data. Misusing a word to evoke an emotional response is intellectually dishonest and manipulative. Entitled is another overused manipulative term.
I am in fact entitled to decide what runs on my computer and spotify is entitled to decide who accesses theirs. If spotify closes accounts for not watching ads and I choose to block ads literally nobody is in the wrong. We are both exercising our respective rights.
At that point it becomes a discussion on what is prudent.
If you arne't in the habit of reading 5-10 pages of legalese when you walk into random stores I wonder why you believe people will read 5-10 pages of legalese before reading the dozens of sites they visit. People care about the terms and conditions exactly when they are informed of being in breach of them and only to the extent that being in breach of them effects their life.
In this case it seems incredibly likely that said users who aren't customers in the first place even if some of them watch ads will just watch free music on youtube.
On net spotify will save money on bandwidth which is incredibly cheap to start with, gain a modicum of new subscribers, and shrink their supply of free users that are their primary source of paid users.
Presumably they are in the best position to figure out if this is worth it and we shall all see.