Strictly speaking, indentured servitude (which is legal) is more like an employment contract than slavery (which is illegal), it's just significantly harder for the indentured individual to get out of it through means other than what's specified in the contract (usually paying off some amount of debt). Therein lies the problem; without strong protections like on Canadia or BAU Steele Tech, it's fairly easy to turn indentured servitude into what is effectively slavery (usually debt slavery), as can be seen in Yammi's conversations about her coworkers and in general with Carver's business. With the latter, I wouldn't be surprised if at least some of what she's doing is just flat-out illegal and it simply hasn't been reported or investigated (I mean, look at what happened to Reaha when she threatened to lawyer up against Carver).
It's important to note that those who do so forcefully are frowned upon even within their own culture; I doubt the wider galaxy looks on it more fondly (the only ones who try to use it by force are typically criminals), and Lessau will even bring it up as a point against potentially uplifting the bothrioc (who also have addictive "cum" but are much less consentual about using it).
"Lobotomises" is a strong word, but okay. Again, this is a cultural thing; the Treatment is not mandatory (though that was after a retcon), and not everyone even within New Texas agrees with it, not to mention that NT is the only place where it's legal. Plus, if Reaha is to be believed, it's possible the only thing keeping it legal there is sexual bribery by the inhabitants.
We don't know whether or not the UGC's negotiations will allow the zaibatsu to continue operating as they always have; for all we know, they could continue as-is or banning that new method might be a concession they'll have to make to join the UGC. Plus, willingness to negotiate with societies whose moral standards differ from your own is pretty much necessary, given that most societies encountered during the Planet Rushes are much less advanced than the UGC and trying to make them meet your standards is better done peacefully than via force.