It might just be a regional linguistic difference but those are two very different things to me.
Statutory rape implies that the underage individual personally desired the relationship - even though they lack the maturity to consent. Suggesting, as the joke does, that Gov. Palin's 14 year old daughter would desire and/or willingly participate in sex with Alex Rodriguez, or any other celebrity baseball player for that matter, is repugnant. We have to assume, as one would with any other woman or girl, that she has no desire or willingness to engage in sexual activity with a stranger unless there is ample evidence otherwise.
Were that not to be the case then wouldn't many a rape charged really just be a "statutory rape"?
In either case, you're correct...the "joke" was utterly repugnant.