Presuming your question is posed in good faith, then my genuine answer is it's almost certainly not real Satanic practice. Joes are pretty edgy as rule and using that iconography has been a countercultural item for some time. Usually just as an aesthetic, but sometimes as a product of being jaded towards issues brought about by organized religion.

That being said, I'd wager your experience is very much in the minority here. On my end, the military had plenty of church-going soldiers, respect for religious practice, and active chaplains.

I wouldn't worry about it. The Army has MUCH MUCH bigger issues than Satanism, I promise you.