i always thought that fool winning with town was incredibly stupid, it basically means fool wins a huge majority of games. the only time they can really lose is by mislynching town on mylo (and even then what's to stop you from forcing nl with mafia for ensured victory unless it's gallis hunter?)
fool "siding with scum" is fool getting lynched, which is fool's primary win condition. nothing in the rules says the fool has to win that way though, if they just play to win with town they can win with their measly 10 points or whatever
your second example doesn't really make sense because fool can side with scum for a joint win but sure, if you have no win condition you can do whatever you want basically, but some roles like fool/lyncher have secondary win conditions by just winning with the town