Why do mods do this?

[zMuffinMan]

I think the obvious reason - that a significant portion of people either don't like it or don't like being blind-sided by it - is a good enough reason to be open about it. You say you don't like role madness; would you like it if you signed up for a setup you weren't aware was role madness? Information about the setup like that (that town is initially unaware of) could arguably be considered an advantage for scum too (any knowledge of the setup is better than none). But mods are open about it because people like knowing what they're signing up for so they can know whether they're going to enjoy it or not.

I think a lesser reason is that town not knowing what they should be looking for means they're just playing blind guessing games. Behaviours that you'd look for in multiball are not going to be the same behaviours you'd look for in a game with a single scum faction and being unaware of what you should be looking for doesn't seem like a fun concept to me. You could argue this should be the case; I would argue it's pointless to play mafia like that. You could argue that town should be able to scum-hunt regardless; I'd argue that there's no real basis for this claim.

I can't think of a good reason why scum need or deserve to have that advantage either.

The point about the vigilante is a good one. I think FakeGod has some decent ideas with regards to that (e.g. gating the vigilante by, say, making it a compulsive role that dies when it hits town or is roleblocked, or something like "N5 vigilante" such that if they come under fire early, town would have to theoretically leave them alive for several days before they could even prove the role). There are probably a variety of other ideas that could be thought up for anyone that still wants to include a vigilante in setup where it's known there's a single faction without having people think it must be a town role.

[zMuffinMan]

So does FakeGod, actually!

[zMuffinMan]

Its towns job to figure out what they are dealing with.

I disagree that this should be a part of town's job. I can't think of a good reason it should be. But if that's the type of game you prefer to play, then I'm not going to argue about it.

So why take that advantage away?

Mostly to make the game more enjoyable for people who don't want it. I guess you could equate it to the difference between a normal game and a non-normal game. Some people like both, some people like one or the other and it's not a bad thing that they can get to choose what they want to play.