In baseball, any player can play any position (unlike, say, rugby where certain positions can only be played by designated players). This is why it has been possible for a single player to play every single baseball position in a single game. There is nothing stopping a team playing their normal catcher as a pitcher or a starting pitcher in any other position.
I think that MLB suspensions are equal for all players because pitchers are not officially registered as being pitchers. Consequently, the leagues do not enforce that a player always plays in a certain position. Therefore, MLB doesn't have the right to give a longer suspension to a player only because he usually plays as a pitcher (even though everyone knows he actually is a pitcher). The equal suspensions for all players is probably part of the collective bargaining agreement between the leagues and the players' union.