If you self vote for the clear, you are game throwing. To self vote is to avoid your win condition: to survive.
By voting for yourself in a situation where you are not suspected of being mafia by the clear, you are presenting the clear with the opportunity to hammer you. Take a game with 7 people alive for instance, where Player A is not suspected by the clear. Player A votes for his or herself in order to allow the clear to hammer. By doing so, Player A raises his or her chances of being hammered from 0% to 16%.
Regardless of Player A's alignment, this is against their win condition. If, in this situation, Player B and Player C are suspected by the clear, then Player A is also reducing the chances of the suspected players being hammered from 50% to 16%, a far greater decrease(and thus a greater gain) than the change in Player As chances.
Now, the obvious counter argument to this is that if the clear suspects Player B, or Player C, they will not hammer Player A because that would be counter intuitive. And I would agree with you. But if this is the case, why then should Player A self vote at all? Why should the clear ask for everyone to self vote and to provide a hammer? Instead, the clear should lead a lynch upon one of the two suspects. In this case, input could be requested from the other live players, providing interactions between players, and more evidence for a lynch. It also reduces the amount of ATE arising from the self vote hammer where players may sway opinions, and a clear given the option of multiple choices may decide to gamble.
If you are self voting for the clear before three way lylo, you are gamethrowing. Mafia should never be voting for themselves, and town should never be voting for themselves.
Potentially, the fool should be voting for himself, but it also makes much more sense for him to vote for someone softing PR.