Assuming that the rumor is accurate...
During one of the past shortages, a relatively small number of people would line up at the local WalMart every single day waiting for the ammo shipments so they could get their limit every single day. Some would even take other people with them so they could get more than the limit.
I'm just speculating, but if BPS was seeing something similar and was feeling the pinch of having large numbers of customers come in and want to buy guns but not being able to get ammo for them, it might make sense for them to try prioritize gun sales over ammo sales. Especially if they are selling out of guns and ammo regularly anyway.
I think the last line of the above paragraph is what a lot of people aren't taking into consideration. If a store is regularly selling out of all their guns and ammo, why would they care if they alienate a few customers who want to buy what they're already selling out of regularly?
Gun Store: We can't sell you ammo unless you buy a gun.
Customer: Why?
Gun Store: Because we need to keep enough ammo on hand so the huge numbers of customers who are buying guns from us can get some.
Customer: I don't like that policy because I want to buy ammo and you won't sell it to me.
Gun Store: Well, if we didn't have that policy you still wouldn't be able to buy ammo from us because we'd be sold out. The only reason we have ammo is because of the policy you don't like.
Customer: Well then I'm going to take my business elsewhere!
Gun Store: You mean, because we won't let you buy ammo from us you're going to stop buying ammo from us? !! ??
Customer: Well, I won't buy guns from you either.
Gun Store: You weren't going to buy a gun in the first place--if you were, you would be able to buy ammo too and we wouldn't be having this conversation. Besides, we're selling guns as fast as we can get them in, so if you want to go somewhere else to look for guns, that might not be a terrible thing all around--especially since you don't like our policies.
Customer: I'm leaving! Don't try to stop me! I'm boycotting you! I'm really leaving! I'll tell people on the internet about you!
Gun Store: Well... Bye. Next in line!