Drilling and cutting rack mounting bracket arms I bought a rear rack so I can finally switch from backpack to pannier. My bicycle is 28" so I picked a rack that can cover 26-28" with "adjustable height screws".  

Upon trying to mount it, I realized that the mounting bracket arms are too long for my frame.  

  

Going back to the store and trying to replace the rack, the guy at the store tells me this is the norm and that I should just cut the arms to the length and drill the new holes for attaching them to the rack myself.  

Since the part of the world I'm living in isn't exactly known for an amazing customer service, I just wanted to ask if it really is the norm or if the guy just blew me off, and I should go to the store again.  

Thanks!
I've got a very similar rack and avoided cutting the arms only by using one screw per arm instead of the two in your first picture (at the rack end). Crossing the arms over and using one screw per arm shortens the effective length still further. You might need to bend them a little bit they're definitely designed for that. I actually ran with only one arm on that rack on one bike, as the left arm interfered with the brake unavoidably. On my similar rack the arms are stainless steel and cutting/drilling seems like a bit too much effort (my drills are probably not sharp enough). 

I've fitted about 10 racks across 6 bikes. Nearly all have needed some sort of modification, or fittings swapped between racks.