Hi,
The standard excuse is the reptile shop represents temporary housing and your set up should be the best it can be - the real answer is it is cheaper for the shop to do it that way.
Your snakes will be better apart in their own enclosures.
Have a search on the forums for the reasons against putting multiple snakes in the same cage - there are very few reasons to say it is a good idea for the averadge snake keeper.
dr del