The National Organization for Marriage is campaigning across the USA opposing initiatives and working to repeal existing laws that would allow same-sex couples to marry. Tim Horton's corporate policy does allow independent franchises to decide what events they will sponsor; however, corporate policies also state that they do not sponsor events "representing religious groups [or] political affiliates".
Change.org is hosting a petition encouraging Tim Horton's to reconsider this sponsorship.
Thanks to epi_lj for the information.
ETA: Tim Horton's has withdrawn sponsorship, citing the "no political or religious groups" clause. Well done! (Note also the new mood.)
And thanks to everyone who signed the petition. Community activism is good.