Someone else may be able to clarify more on this matter, but here is my understanding:
A royalty interest that is taxable is not the same as a mineral interest that is taxable. In Tarrant County (and several other counties in Texas I am familiar with) we do not tax mineral rights. We DO tax royalty interest (so if you own your mineral rights, and there is production, you are assessed taxes against the royalty interest aka production of minerals). This creates an issue of real property vs personal property. In Texas, mineral rights or mineral interest is considered "real" property, as the minerals are "in place". When the minerals are developed and brought to the surface of the earth, and you have a right to income (royalty interest, overriding interest, working interest, etc) it is now "movable" and no longer "in place". These kinds of interests are taxable as personal property, since personal property can be "moved" (easily), unlike real property.
I'm sure everyone is aware of this but I will say it: Every state is different. The kinds of taxes enacted in each county may be different. Some counties may tax minerals, some may exclude taxing royalties. Tracking the ownership for minerals can be very difficult for a county to do, as people die and a single residential lot with severed minerals could have 100 owners in the mineral estate, each with different kinds of ownership. So getting copies of the division orders from the operators and taxing only royalty interest owners is more than likely much easier, and common place (in Texas - from my experience).
Camille - I have been watching and seeing similar sales lately as well.
Here are the Texas rights of redemption rules regarding REAL property:
http://www.window.state.tx.us/taxinfo/proptax/tc06/ch34b.htm
Texas A&M Real Estate Center guide to foreclosure and tax sales - 6 months redemption on all property sold OTHER than "real" property. I believe this includes taxable royalty interest and similar interests.
http://recenter.tamu.edu/pdf/652.pdf
Texas Oil & Gas Real Property vs Personal Property (See page 34)
http://www.beardenlawfirm.com/Texas%20Oil%20and%20Gas.pdf
Definitions of "Real Property" for Texas
http://www.pandai.com/faq/PTC_Section_1.04%282%29%28F%29.Definition...
Here is the Texas case involving the purchaser of a surface property at a tax sale that sued to gain the royalty interest that was present at the time of sale:
http://www.leagle.com/decision/in%20txco%2020091208591
http://www.bf-law.com/casenotes/1324595827314.pdf
Here is the case involving someone who bought a royalty interest at a tax sale and sued to get royalties held in suspense and lost.
http://caselaw.findlaw.com/tx-court-of-appeals/1092409.html