So long as those restrictions were in place when the contest was first announced, I see no problem except the gun store guys did not read the rules or thought trhew rules did not apply to them.
Saying that Staples is equating gun shops with illegal drugs is going way overboard. A more accurate statement would be one to the effect that Staples not only does not support/condone illegal stuff, they are prejudiced against certain products/industries because the find them "icky".
If I was a national corporation running some sort of contest I'd make Rule #1 read: No idiots allowed to enter this contest. Then I could pick and choose who to call an idiot. (I used to write down the names of all the people and businesses that I thought were, for any reasaon, idiots. I found it is easier to write down the names of those that are not - keeps the list short and sweet.)