What Im unsure of is if there is a bet on the river are you forced to raise or can you just call?
Thanks 
First, can someone point out where in the WSOP rules it says this? The only thing I can see are rules 39a and 99 which talk about soft play, but I don't see anything which actually describes soft play. Possibly this falls into the "what other reason than soft play could there be for not raising on the river with the nuts?", to which I would respond "People make mistakes all the time". I've also heard of some ridiculous situations, for example, the board is T,J,Q,K,A (Broadway), with no flush possible, and the last person to act got a penalty for not betting.
Now, on to your question - this is a very specific situation. In order for the penalty to be considered when there is a bet, the player with the nuts must be able to raise without that raise affecting the possible calling action of anyone else.
For example, 3 players on the river, players 1, 2, 3, and act in that order.
Example 1:
Player 1 checks
Player 2 bets
Player 3 (with the nuts) may now call, since they could argue that Player 1 might call if they call, while Player 1 and Player 2 could both fold to a raise.
Example 2:
Player 1 bets
Player 2 calls
Player 3 must now raise since there is no possible loss by putting in a raise.