Is compatibilism an excuse to punish criminals even without free will?

If we assume no free will, the notion of "excuse" doesn't really apply. In a world without free will nobody opts for excuses, they just do what they were predetermined to do mechanically. People have no agency, in any meaningful way.

So, if "wanting" corresponds to some mental state, people will be predetermined to have a single specific state about it (" wanting" to punish or not punish criminals) at any given time. They will also be predetermined to punish or not punish criminals - they can only do what they end up doing.

Thus, if we assume no free will, there's also no meaningful sense to say that "people hide some true intention". Nobody has any intention, same like a rock doesn't develop any "intention" to fall to the ground when it does so, it just falls.

As for the question: since very few believe in any lack of free will, society will continue to punish criminals anyway. It's not like jail time and legal punishments are in even the remotest real danger to stop because of philosophical discussions about free will. So that can't be what's causing the tiny minority of people interested in such matters to advocate for combatibilism (and thus, no case of "true intention", as if they're sadists who don't want society to skip punishing people who crime without free will).

Rather, a more obvious intention (again, assuming free will exists, and people do have intentions, even if they themselves believe there's no free will) is to make a universe that's deterministic (and thus traditionally understood to lack free will) compatible with the experience and intuitiveness of free will (hence, compatibilism).

Free will and lack of free will is not relevant, I think.

The argument about predetermination is also wrong, because even if something (or everything) is predetermined, they follow certain patterns, e.g. when someone asks a question, someone else may answer it, and when stones fall, they will fall according to the gravity, etc. So, your "intention" is also one of these patterns (possibly an emergent one, but still it is one either way).

(In my opinion, criminals should only be punished if they actually committed the crime (sometimes mistakes can be made, so you should make sure) and are likely to do again and the punishment is sufficient to avoid repeating it, or if it will reverse whatever problem they caused (e.g. if they stole something, to give it back, or someone will steal it back). (In some cases all of these criteria will apply, but not always. Also, revenge does not generally solve it.))

If we don’t have free will… why do people need an excuse to punish criminals? They have no choice!