1. The LBS did not install the BB, but they did assemble the bike out of the box and should have tuned it up (including tightening crank/bolts). I do not know of any bike shop that expects customers to do this right off the bat. If you take the bike out for a test ride in a parking lot and the crank arm falls off (seen it happen), is that your fault or the LBS for not properly setting up the bike? The bike shop should provide a well set up bike, that is one of the advantages to buying from an LBS over something like bikesdirect.com. If they don't set it up right, they should be responsible.

2. 185lbs is not a weight concern for anything on a stock Bowery. He could add 70 lbs worth of gear and still be OK. He would have to tune things up a little more often, but he would not ruin a BB after a month and a half.

3. The only work that should be required for the first month and a half of riding that bike is keeping air in the tubes and lubricate the chain (frequency depends on conditions). Every LBS I know of says that after about a month or so, bring the bike in for a follow up tune up. What the LBS is looking for here is mainly cable break-in, but they should also do another loose bolt check over the bike to see if anything is working its way loose. This is where the OPs bike is at. Regardless of if he brought it in for a follow-up or not, mechanical parts should never fail after this amount of time (again, excluding high milage [500km is not high milage] or abuse).

I am not saying it is the LBS' fault - it could very well have been a bad bottom bracket from the factory. However, even if the blame is on Giant, or whoever made the bottom bracket, he should still go to the LBS and ask them (again, nicely) for help, because the LBS should take care of his bike, and then take care of it with Giant. When something like this happened because a new part went bad, we replaced the part and then would report this back to the distributor and work out some arrangement, be it reimbursement or replacement. The LBS will have a much better relationship with the company than you will, and they can get much faster and more positive results.

Finally, you are correct - this is a very cheap part. Both the LBS and Giant do not want to turn you off from their business over a cheap part like this failing prematurely. The OP wants his bike fixed, and the businesses want you riding happily again. People coming in for handouts suck, people coming in for fair business dealings are great, and made up most of our regular customer list. Something was not right on this person's new bike, the LBS sold that bike, so they should serve as point for fixing it.