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.
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.