It's not "buy" by itself, it's "buy a film" or "buy a TV series".

The word "buy" followed by the name of a service (say, "access to films") can indeed carry the meaning you describe (so in that example "buy access to films" is the same as "renting films") even if it is an unusual wording, however when the word "buy" is followed by the name of a good, not a service, (i.e. "buy a film") it is interpreted in trade, legal and common terms as acquiring ownership rights to that good.

Granted, IP law is a big bloody mess and Consumer Rights in places like the US are pretty much Fuck-You-Plebe, so legally in the US who knows what levels of misleading contractual terms and one-sided post-sale of imposition of new contractual terms via EULAs towards retail customers are legal, but in both common usage and trade, in pretty much all areas but those covered by IP Law, "buy a good" means buying a product, which is something else altogether than "buy temporary access to a good" which is the meaning Sony is using.

Generally in Consumer Law, these things tend to boil down to whether a normal individual with no legal expertise could be reasonably expected to understand the terms of the contract with the meaning Sony claims or not, though in the US, with the kind of Consumer Protection laws it has, run-of-the-mill (not rich, not lawyers or judges) consumers are most likely legally fucked, de facto and quite possibly de jure