Why they can be forged broken?
Because it'd be hard to put limitations on the forge to prevent flame breaking in one circumstance while also not restricting creativity in another circumstance. It'd be hard to draw a line and catch every case I'd imagine.

Why they can't be fixed?
Because no tool exists to fix it. The only way I can think of is by directly changing the database entry for it or making a tool that does that. And then the first issues ties into it. How do you look at these broken flames on a case by case basis and decide what is fixed and what isn't? Who does that? How does it work?

Bit off topic but still related:
Really there shouldn't be an issue of users ever getting a broken flame and being surprised that it's broken. The rules in the flame particle texture board are there so that you get a good idea of exactly what you're buying. You should also always look up flames that you are buying or making deals about in the flame browser. With all these measures - if you receive a broken flame there can't be anyone to blame but yourself really. Therefor the only people getting broken flames should be those that forge them or those that, for some reason, specifically want them for some reason. (Eg, ishi collects broken flames iirc)