Customs discovers what is in it...the DEA/local police follows it up...
I'm not trying to single you out at all 40AmpstoFreedom, I've been hearing this from a few people and quoted you real quick.
The role of an agent sorting through suspicious mail is that of verifying that what's being shipped is dealt with accordingly pertaining to any clearance lists that customs agent is required to decipher from.
What gives them the opportunity to follow anything if a package becomes seized property, I'm sure sending out notification gives them cover if anyone were to wanna sue them, but anything beyond that would be like entrapment considering the package stopped by them would have no possibility of ever being received if it weren't approved.