As you are already aware, the two derailleurs are different. Obviously, Shimano thinks the triple specific front derailleur is superior for that use or they wouldn't bother to market it. Honestly, your bike shop people undoubtedly know that too.

So what are you going to do now? I think that you're in the right and I don't think that you're being petty. If it was my bike I'd take one shot at getting the LBS to put on the right derailleur. If that didn't work I'd give up, get one to stick on myself, and put the issue behind me so that I could just ride and enjoy my new bike.