Unfortunately ,things like this do happen ,unless there is a adequate quality control (there must always be someone overseeing the finished work,then they would have noticed the loose bolts).The dealer obviously has a work shop manager but I have seen indies with stricter control and checked all work on the job sheet.These are professionals ,eventhough they are not official dealers.
Your mechanics attitude to what happen (could have been disastrous,had the last bolt fallen off),deserves another chance and lets hope he learns from it and double checks all work ,before letting car out.