I will agree with you on the blue loctite use, but I disagree that there should be any used for this problem.

Is it the bolt has not been torqued sufficiently to hold the bars in place, are they the right fastener, can the stem with tightened bolts firmly hold the handlebar in place? If the "slot" that the bolts compress to hold the bars in place doesnt have at least some space it wont hold the bars. Many times a "dry" fastener in aluminum or steel starts to gall and doesnt torque properly. A little bit of some oil or my favorite never seize wil fix that problem.

BUT.... since this is a safety issue I am sure that REI would be more than happy to make the correct adjustments to make sure your handlebar is secure. Let them do the job that should have been done when the bike was assembled.

-SP