I actually paid $29 to use SNOW leopard, the mac already had leopard on it. And if/when i pay $29 for Lion, I will have paid $58 (unless somehow $29+$29=/=$58 as you apparently seem to think) to upgrade from leopard to lion. You aren't paying any more, nor any less than I did, so what's your point.
The more I think about it, the less of a shocker it is to have to pay for the SL upgrade first.
Apple is pricing low and calling it an "upgrade pricing" each step. The double upgrade is still cheaper than if they charged over $100 each OS. If they did, then of course you should expect any OS to be upgradable.
There are applications on for the mac and I'm sure Windows that have upgrade prices. Some, but not all, pro-rate the discount based on the latest version you own. The more recent the version, the more discount. So this is not unheard of.