This is speculation on my part but one possible explanation there is some issue with the graphics and for now they don't want people giving bug reports and feedback based on a variety of FOV's used so they disabled our ability to mess with it. It's a troubleshooting step.
Somewhere in that "Alpha 18 Dev Diary!!" thread which has too many pages to find anything now, MadMole said something about how he thought the narrow FOV looked better and he didn't like seeing screenshots people took of the game at wider FoVs. He thought it made the game look bad.
I don't really understand that train of thought myself, because there are a lot of graphic options that, if tweaked, could make screenshots look bad, so why single out only FOV to lock down? Besides that, I think it looks more natural wider because in real life we don't have tunnel vision. We have slightly blurred peripheral vision too, and the wider FoV mimics that.
At another point in that same thread, he (or maybe one of the other devs) mentioned the wider FOV options were removed because of occlusion culling - on wider FOVs, there was the risk of seeing blocks disappearing on the edges. Someone pointed out (in more technical language than I'm about to) that the angle for the occlusion culling need not be fixed. It could vary according to the FoV which would solve that.
The convo didn't get much further than that if I recall because MadMole got tired of seeing all the posts on FOV and banned anyone from discussing it further. He labelled it 'off topic'.
I have my personal suspicion that maybe it has something to do with them not being able to fix that hack some lame losers were using in multiplayer to see under the ground. Of course removing the slider won't stop the exploiters because they will just use the console command.
I really hope the devs stop being so hard-headed about it. I suffer from motion sickness at the default field of view, and having to remember to type in the console command each time is annoying AF.