I wanted to find the exact reference but I couldn't but the point about "free odds" not being really free is summed up in this paragraph on Wikipedia:

The only exception is the "odds" bet that the player is allowed to make after a point is established on a pass/come don't pass/don't come bet (the odds portion of the bet has a long-term expected value of 0). However, this "free odds" bet cannot be made independently, so the expected value of the entire bet, including odds, is still negative.