The existence of the Watcher would not scare off the Mafia’s factional kill, because that can’t be Watched. It could, however, scare off the Outrider, who can be Watched. So there’s a reasonable chance that a single-outed-power-role would be targeted for a kill but not Outridden, which makes protection a reasonable choice.
Why do you think that? He had already declared himself as the EMT, and had said publicly that he would be self-protecting. That takes out all the luck from the Outrider targeting him, and discourages the Mafia from making the attempt. You’re positing two parties targeting him, one having absolutely no effect, and an unknown third party making the other kill: That seems like a lot of extra assumptions to make, when we can account for both kills using mechanics known to be in the game.