The difference there is that essentially MLB is the only game in town and Costas doesn't run a private business where he offers "objective" analysis of an entire field. Had Meltzer taken the gig his credibility should have been shot, but most probably would have ignored simply out of blind love for NJPW. This would be more akin to Costas owning Baseball Prospectus and doing announcing for the San Diego Padres while still purporting to be delivering unbiased analysis of the Padres product in his Baseball Prospectus work.
What about all of the team owned networks (I'm thinking YES, SNY, NESN as I'm on the east coast) that employ sports writers or sports radio hosts to be a part of the broadcast/network. Occasionally those guys get flack for objectivity issues (by media talkers), but its never talked about as this disgrace to the profession.
I think those being critical are reaching on this one, especially within the current sports media climate.