Why are my heat shields on my 04 Honda Accord loose and vibrating?

So, I had all of my brake pads, rotors, and calipers replaced at Firestone. About a week ago this horrible rattling was coming from my front axle. I assumed they forgot to tighten something, so I brought it back in. They said it wasn't their fault, but to fix it, it would be $600, because of all the labor to get to the part! They said the screws were loose, so the plates were vibrating. They said it wouldn't damage anything if I drove it like that. It is soooooooooooo irritating though. Something needs to be done. It happens at 1.5-2.5rpm's. I just have to accelerate faster or let of the gas. I think the sound is getting worse too. Anywho, could the mechanics have loosened this part on purpose so I would come back, or is it just coincidence that it happened a week after I brought it in? Also, is there any way to tell? Oh, and is it really a $600 job?