Indeed, patent-license negotiations may often fail to approximate the conditions presumed by the classical economic model of efficient bargaining. See, e.g., Sag & Rohde, Patent Reform and Differential Impact, 8 Minn. J.L. Sci. & Tech. 1, 36-37 (2007) (patent holder may have asymmetrically better information about the scope and validity of the patent) ~ Kimble v. Marvel Enterprises, Inc., 2015 WL 881759 (U.S.), 45 (U.S.,2015)
In Brulotte v. Thys Co. the Supreme Court ruled that, when patents are sold in return for a royalty payment, the purchaser was not obligated to continue these payments beyond the expiration date of the patents because doing so would over-compensate the seller of the patent and improperly extend the patent monopoly beyond the intended time limit.
In other words, it is patent misuse to try to extract royalties for use of the patent post-term. That rule is now being challenged. For a summary of the issue in the case, see http://www.oyez.org/cases/2010-2019/2014/2014_13_720