The policies are more symbolic than anything, they would signify that the company doesn't condone such behavior. However, I think such policies should, by law, kick in after conviction and appeal processes have finished, when incidents are criminal in nature. If a person is not convicted, these policies should not apply. Too often, they apply upon arrest.

The difference is in the burden of proof. To be convicted in the criminal system the burden is "beyond reasonable doubt". In the civil system it's lower: "preponderance of the evidence". In the eyes of an HR department it's lower still. Theoretically, if you're arrested there's already "probable cause" - i.e. some evidence. It doesn't seem that weird or unreasonable that HR would act at this level. Does it suck if you didn't do anything? Yes. But people get fired for much less reason (or even no reason) all the time.

There are plenty of things that are not illegal that would also not be acceptable in many workplaces. For example: drinking on the job, coming to work without a shirt on, giving everyone a nickname (e.g. "beardy") and exclusively referring to them by such, etc. You don't have a right to a job, so companies can fire you for any of these reasons. It would be weird if committing a misdemeanour or felony on the job meant you would get to keep working until your trial is over, but you could be fired on the spot for having a beer.