They are not supposed to fire you for not getting them, only for not asking. That's the "official" policy. But they can work around that by coaching/writing you up for every single instance you're overheard not asking. So if you ask 100 guests 100 times, but then you fail to ask the 101st guest, they could make that a coaching if they wanted to be nasty about it. If you don't slip up there, they can find other reasons to "officially" write you up which would eventually be a firing. The truth is, they can fire anyone they don't like because everyone can be nabbed for something. Or they can just threaten to fire you if you don't sell enough REDcards, which is enough to scare people even if they don't actually mean it.
Target is the worst when it comes to the pressure and stress they put on their low-wage employees over selling credit cards to people. I know a lot of chain retailers including Walmart, etc. like to push their store cards but if I had to rate the manner in which they did it Target would be at the bottom. Where I work right now, Lowe's, there is no one yelling at you for not meeting a quota. At Target, you could very well be fired for not meeting the quota. It's disgusting, especially when employees don't get paid enough to be treated that way.