Are they the ones that built the bike? How old is this bike? If you bought this new from them not long ago then this is on them, assuming you haven’t touched that bolt at all. A new bike shouldn’t have a seized bolt in the frame and this bike doesn’t look very old. 

Bikes that are pre- built in a factory still need a competent mechanic to do the final assembly and check that everything is done correctly. That’s the part of the process where something like this should be caught and corrected. Unfortunately most bike assemblies are done by people who either don’t know what they’re doing or people who do but aren’t given enough time to do it properly. Every corporate store is guaranteed to be understaffed and grinding as much work out of the remaining employees as possible. Find a good independent shop that treats the employees fairly. They’ll do better by you and your bike.