But are not investigations typically under some reasonable suspicion that a crime had occurred?

Reasonable suspicion is required for a terry stop but that does not apply at a border crossing. Everyone can be stopped and questioned and searched at a border crossing.

An ”investigation” is just a fancy word that broadly describes the type of information collection that law enforcement does. Yes, it is typically about things they think are crimes. Because that’s their job. But there’s not some legal hurdle they have to pass here, the word describes their work.

they of course arent police and arent investigating anything

instead they are harassing

It is illegal to destroy evidence even if you weren’t guilty of the crime that was allegedly being investigated.

If someone accuses you of a crime, committing a different crime is probably not a good move, in any situation.