As a point to mention, when May was looking for work, the clothing store she tried to apply at wouldn't even accept her application. The owner was comfortable being openly biased and discriminatory, which says to me that there are only limited legal protections on AI rights. To parallel, in a lot of states in the US, you can be openly fired for being gay, there is no law against it. Even in some cases working towards laws to protect people's right to be homophobic jerks. So yeah, gay people have rights as human beings... to a limit. On the other hand, if you refused to hire someone because they were black and openly said so, you'd be sued eight ways from Sunday. That's probably the status of AI rights at the current time. They are recognized as sentient beings, so it's illegal to say, abuse them, force them to do something against their will, etc. But they're still second class citizens where it is legal to discriminate against them.
Which is a likely explanation as to why some robots wind up in the fighting arena to make an income. Because employment opportunities are bad enough that they are forced to illegal activities just to keep running and powered. Possibly even for shelter, the way Bubbles lives at the rink. Basically, technically legal but still considered undesirable by a large section of the population. Probably unless they are in what are considered 'appropriate' roles. I don't think this is all that far off, since there have been other parallels between how AIs are treated in QC and how LGBT people are treated in the real world.
Of course, some of them, like Punchbot, probably just do it because they enjoy their work. I mean how hard is it to find other work when all you want to do is punch things?