Some Wild Guesses
(1) M$ is the patent(s) and copyright holder on the OS. It doesn't seem the place of a hardware manufacturer to take up the cudgels on their behalf.
(2) They believe that M$ should pick up the tab for the campaign?
If Microsoft hasn't been fair enough with the PC makers in the past regarding the price of their OSes, they could be unwilling to invest their money in rebuttal of Apple's ads.
(3)They have their own issues with M$, such as discontinuing XP.
(4) M$ has bullied them in the past, and this is revenge?
(5) Last but not least, the PC manufacturers have (IMHO) more to gain than lose by having MS fall from absolute dominance of the OS market. As it stands now they have no leverage whatsoever.
These are only suppositions.