This allows me, as a Facebook user, to complete my profile and declare that I like something, even if the marketing that the associated Page is doing isn't something I want to see.
This ensures that profile information remains accurate, which is critical for ad targeting - people don't start unliking things they actually like, just to avoid annoying marketing. The interest graph remains intact even when brands make poor marketing decisions.
Maybe I've been on the other side for too long, but I equate liking a page with opting into their facebook marketing content, and I unlike if I want to opt out.
I'm not sure why anyone would assume a very casual social "hey I like X" is equivalent to "subscribe".
In normal social circles there an expectation that the you want to hear about things you 'like'. For instance if in real life I have a friend who 'likes' classical music then when I hear something about classical music I will talk to them about it.
Also like is not like subscribe because the idea behind subscribing is that you receive everything. For instance if I 'subscribed' to a magazine I would be rather upset if they only sent me every 7th issue, or only sent me issues when the newspaper had not arrived.
People commonly associate/link their personal identity with media and products, certainly.
Twitter is more obvious semantically, but heavy Likers of brands on Facebook already understand the implication, and increasingly so will everyone else, regardless of the terminology. For me it's basically to the point where I only Like things that are owned or run by people that I actually know.
I never liked anything on facebook (via a 'like' button) but I do have several likes.
Before the facebook feed , before even the facebook wall, the personal information field of your profile was a text blob. Over time facebook parsed this information and matched it to pages, giving me likes.
Now i get updates from films in my feed because 7 years ago I added them to a text field called 'movies/tv'
Doesn't bother me because I hardly use facebook anymore. But it is interesting to think how it has changed.
One thing to note, when you said "my personal feeling is that if I follow a brand it's because I want to see their updates", the tricky think about facebook is that you don't "follow" a brand like you do on Twitter, you just "like" them. In my opinion, it's a small but important distinction. Just because you enjoy a certain brand doesn't mean you actually want to know every little thing they're doing.