I know a lot of folks have weighed in on this, but I thought I'd add my thoughts as a long-term wrench.

1. It would have been helpful if the LBS had advised you to check your cranks and pedals for tightness after 50 miles or so. NOT touching the pedals is not necessarily a good thing, but it's the LBS job to advise you, and to assume you don't know automatically to check. It's similar to the written warning you will typically get on your repair receipt when you have work done on aluminum wheels on a car.

2. One thing I have seen stress pedals, cranks and BB's is what I call the "Roy Rogers" method of mounting/dismounting. That is, placing your foot on one pedal, pushing off and then swinging your other leg over the bike. Dismounting is the reverse process. The problem is that you can exert force in excess of your body weight, in one spot on the rotation, over and over. Heavy persons especially can stress things enough to cause problems.

Yes, it's quite possible to do the above without damage but I saw a high correlation between that method and various crank/bb problems. 90% of the pedal and crank arm problems I saw were with the left side, which is where the great majority of folks mount their bike.