Liddy has been a fundraiser for McCain's various campaigns and McCain accepts that Liddy is his "old friend", something Obama has never called William Ayers. The other difference is that Liddy committed his heinous crimes while serving as a Constitutional officer of the United States, his salary paid with your tax money. Furthermore, Liddy has voiced no valid remorse and, unlike Ayers, has not been rehabilitated and has never contributed to the greater good of society.
A lot can be said of the nefarious tactics of William Ayers, and certainly nobody commends them, but his overall mission of ending the war in Vietnam was certainly intended to be in the national interest. Liddy's criminal tactics, on the other hand, were intended to serve not the national interest, but the interests of his demented drug-addled overlord: Dick Nixon.
Congratulations, Mr. McCain, you have your very own terrorist!