I'm guessing you were connecting in LHR from some other destination, i.e. XXX-LHR-DFW?

The UK charges a higher APD (tax) for business/first class, often called the luxury tax, so for any SWU requests on ex-LHR itineraries AA will take your credit card info and charge the higher taxes if the upgrade clears.

This tax isn't due if you're just connecting through LHR.