The reason neutrals should be allowed to buy in clan and omni shops is simple : neutrals are not their enemy, neutrals are more like peaceful foreigners and since when merchants don't sell to strangers. Imagine how stupid it would be if you took some vacation to a foreign country against which you're not at war and they wouldn't allow you to buy anything.
Also omnis and clanners can buy in neutral shops so I don't see any reason we couldn't do the same in theirs.
And the more customers merchants have the more money they make and the happier they are (don't tell me omni and clan merchants don't care about money, all merchants are the same when it comes to money, wherever they come from).