There doesn't seem to be any rule saying that robots without owners cannot remain free, nor does there seem to be anyone tasked with rounding them up, so that's a nice point of divergence from other stories like this.
interesting thought.
we know that ponko was owned by a house keeping company of some kind, and they would rent her out to various places. she was a nurse for a while, and probably had many other jobs, but it has never really seemed like her intended purpose -- which is why she's seen as useless and clumsy. was she repurposed to be a house keeping robot after the war? we also know that ponko is set to be destroyed after this current job ends.
my theory is that there is some sort of government agency that is responsible for collecting and destroying humanoid robots, but rather than actively looking for them, they probably provide incentives to companies/people who own them to turn them in, or replace them. so eventually they will all be rounded up, or break down from lack of parts. we also know that humanoid robots are no longer produced, so i imagine there is some regulations in place preventing new parts/models from being made.
if the government is aware that humanoid robots can become sentient and/or disobey direct orders, it's surprising that they aren't more aggressively trying to destroy all of them. there has been a bunch of passing mentions of humanoid robots no longer being made because of all the problems that came along with them, so even the general population is aware that they have been deemed problematic or dangerous.
we still have so much to learn...