Ball joints-S10...

I took my S10 to a mechanic today to check the alignment and possibly
replace the idler arm. The mechanic said my ball joints need to be
replaced. I had them replaced not too long ago. I was amazed at the
amount of play in the wheel when he rocked it back and forth. They
were going to charge me $550 to replace the ball joints. I told him
I'd have to think about it. So I took my truck home and checked out
the ball joints myself. It appears that bolts that hold the lower ball
joint to the control arm are loose. And this is what is causing the
play in my wheel. However, the mechanic failed to notice this...or
perhaps they were loosened on purpose. At any rate, what would cause
the bolts to work loose besides someone loosening them? Anybody got
any recommendations on what I should tell the mechanic?