Your dealer is not lying to you.
Federal law prohibits "manufacturers, distributors, dealers, or motor vehicle repair businesses from knowingly making inoperative, in whole or in part, any part of a device or element of design installed on or in a motor vehicle in compliance with an applicable motor vehicle safety standard." [49 USC 30122(b).] The National Highway Traffic Safety Administration has made it clear that this provision applies to customer requests to disable TPMS systems or install conventional valve stems in place of TPMS sensors. [See,
http://www.tirereview.com/nhtsa-clarifies-make-inoperative-provisions-of-tpms-regs/ and
http://www.autoserviceprofessional.com/article/95838/profiting-from-tpms-service.]
Your dealer would be subject to substantial fines and penalties for disabling your TPMS system or any part of it. Why would they risk that?