We talking a Georgi Markov scenario here? NSA thinks you might be a danger to Federal power, so they sneak up on you and implant it without your knowledge. Or would this theoreticly be entirely above board. "Welcome to the United States. Here's your temporary student visa and *pfft* subcutaneous kill switch. Please enjoy your stay. Just don't over-stay your visa, because that kill switch is automatic. Thank you, and have a nice day."
I'm pretty sure the former violates several vital human rights, and the latter violates several vital Constitutional provisions, not to mention tipping the eva-doer where to apply the scalpel to dig that bad-boy out.
Frankly, I think someone's been watching too much THX-1138, Logan's Run, and Demolition Man.