Because it has the appearance of being a cheat, an inbuilt workaround for all players to use and one endorsed by FD.
I'm still trying to figure out how it has the appearance of a cheat, when FD has declared being docked in a station a safe zone, no matter what mode you're in. You're just P.O.'d that the player in the station won't come out and fight. And there is no way you can force that.
If FD eliminated Solo Online mode, a player in All docked at a station could just as easily log out, and log back in with a chance of getting into a different server instance. Then you disappear. That's functionally the same thing as switching to Solo mode. Forcing everyone into All mode won't prevent this.
In other words, every situation where the game currently allows switching from All to Solo Online mode (and it won't happen in combat AFAIK), is a situation where a player in All mode can just hop servers. So why aren't those players considered cheaters?
It's a game, it's going to work in a certain way. I think you're going to have to get used to the idea of Solo Online mode, because there is no more potential cheating in that mode than any other. Everyone has the same opportunities in this game.