Hmm. Interesting....
I'd say the differences are, I honestly don't think Kendrick really meant much by what he said on Control, regardless of how hip-hop took it. Plus Kendrick can't (and doesn't want to) really do damage to their brand. Bigging up Kendrick doesn't mean anythng really other than it makes a nice soundbite. Hell, it might even diffuse the situation.
Doing a song is different. Drake stands to lose by helping 50 succeed just as much as Wayne does. 50 called Drake soft basically and and has sent shots at Cash Money, as well as being an overall bully and nuisance for the entire industry. Nobody gains by helping him except him.