I think it is a very valid point.

Sheehan is making what she claims is a moral objection, and possibly allowing her dead son to be exploited to do it. It's none of my business, but she has made it public and so left herself open for inspection.

If Sheehan is shilling with a political agenda, I think she is acting in a partcularly unattractive and unethical manner. Not only is she popular with America's liberals, but she's all the rage on Al Jazeera these days.