I don't see any problem  with a gift shop near an exhibit or placed so folks
 have to enter or exit through one.  The operation of a shop is a legit thing
 for a museum, after all. Some places even place the shop so it can be visited
 with out paying the admission price for the museum.
However, It is not a good practice to mix  gift shop items with collections.
 It seems to me there should be a clear dividing line between the two.
 Properly placed a gift shop can supply an extra pair or two of eyes to keep
 watch on some part of the exhibit space.