Many years ago, back when I was a young intel officer, I asked for the reasoning why there were any groups of people **at all** that were off limits to being recruited. If HUMINT is all about "placement and access," don't we want to recruit people who have... placement and access? Like, this ENTIRE LIST <whatever it was> of prohibited categories of people?
Reasonings:
1) protect people who AREN'T sources. "I couldn't POSSIBLY be a CIA spy, I'm a ***."
2) protect against double agents. Sources who are close to the target run the risk of "going native," and that's especially true of outsiders who earn their way into communities.
3) ...I had something else when I started typing but I've lost it; if I remember what it is later, I'll post it