First of all, it's not a matter of what the IRS wants, or what would allow them to collect more tax. The IRS just follows the tax laws. It's working the way Congress designed it, for whatever reason.
Secondly, the primary purpose of the RMD is to reduce the amount of money that you have in the IRA, so that you can't keep it all there forever. Taking out after-tax contributions helps to reduce the balance.
