It's just inherent to the class, at the end of the day. Sure, a BiS 2h is 23 coins, but it's really just one weapon. Two 1h BiS is two BiS weapons, so...you're buying two weps.
You can look at it this way, I guess, though : Take a DK for example. Three tanking specs, three dps specs. If you want to buy the BiS weps for a DK, for either two dps specs, or a tank and dps spec (assuming you dual-spec), you're either going to be buying two 2h'ers, or 4 1h'ers, or a 2h and two 1h'ers, since each spec requires different weapon specifications.
You can't just buy 1 2h and call it good, since there are different BiS weapons depending on the specs you choose.
And god help you if you want to ever try / need to change your spec as a DK, you're gonna need to buy another 2h (And no, Smourne isn't the be all end all for DK's :p It's actually not that great for them, in case someone was going to mention that one ;) )
Or a Prot warr, you'll need to buy, at a minimum, 2 BiS items, your 1h and your shield, and if you want to dual-spec, again, that's two more 2h'ers.
When you look at it from those perspectives (just as examples), I think the coin shop pricing is more fair overall than you think.
Again, you're paying to skip the grind to get a BiS item, so 'fairness' doesn't really factor in, at the end of the day.
Technically, they don't need to offer a single item, or even have a Coin Shop, donations could just be merely that, donations, with maybe the q-skipping offered as a bonus.