Interference and Obstruction

I just don’t get Baseball Interference and Obstruction. Last night, A-Rod motions with his hand, knocks the ball out of Arroyo’s mitt, and is eventually called out on interference. Jeter, who had rounded the bases, is sent back to first even though he would have easily reached second without the interference. Last year, Mueller obstructs Tejada as he rounds third. Tejada is awarded third, allowed to advance at his own risk, and eventually ruled out at home.

Why is there a punitive penalty for interference, but no punitive penalty for obstruction? In the one case, the offending team is punished by losing a base that it would have clearly reached without the interference. In the other case, the offending team is not always punished - the umpires rule based upon their determination of the expected outcome without obstruction.

It seems to me that there should be a punitive penalty in both cases. As it is, there are cases in which it is to the fielder’s advantage to obstruct baserunners.