Not a bad point, but castration is irreversible (what if they are found innocent) and rather barbaric (so unlikely to gain the support needed).
What about chemical treatment? Would modern medical technology allow (removable) implants that remove an implantee's sex drive? While we're at what about one that dampens aggression? They could detec hormones like testerone and release inhibitors. Is this feasible?
Nice idea about the implants, but unfortunately, that would encounter similar (although less blatant) problems as imposing castration. One could argue that an anti-aggression or anti-sex-drive implant would remove free will from the equation (a-la "A Clockwork Orange").
The current penal system is based on the idea of rehabilitation where possible. If you had a reversible implant that stopped or reduced the likelihood of you offending, whats to stop you repeating the crime once your sentence is up and the implant is removed? While a fitting punishment, reversibly removing someone's sex drive may not encourage reform as much as suffering the indignities of a prison sentence.
Now that I've made a "serious point", I'd like to offer my confusion at why an organisation concerned with liberating animals would castrate one. Maybe next we should start controlled extermination of Cows because they cause air pollution.