Romney, Huckabee and Palin’s endorsements draw more fire because the three of them are presumed candidates for 2012. Thompson is not.
Sorry, that excuse doesn't hold water if we look at past elections.
Fred Thompson was a "presumed candidate" for President during the 2008 election cycle and his history of endorsing RINOs drew absolutely no ire on this forum. Everyone else that endorsed the kinds of people that Fred did caught hell for it. When the NRSC ran ads promoting socialist Linc Chafee in the 2006 primary, a bunch of freepers vowed to "never again" donate to them. Let me know if you can locate any posts where freepers vowed to "never again" donate to Fred when not only endorsed Chafee, but personally came to Rhode Island to campaign for Chafee over the conservative challenger. Nobody retaliated against him in 2008 for helping elect a bunch of RINOs.