I believe that if you have a werewolf follower, such as Aela, and if you take on Beast Form then they should take on Beast Form with you. This only seems logical as followers already draw weapons, sheath weapons, and sneak when you do. So, why wouldn't they take on Beast Form if they were capable? How cool that would be to have you and your follower clearing out a dungeon together both as werewolves! I wonder why this is not so?

probably they didn't think many people would use such a feature, so they never went through the process of scripting it to happen

Not only don't they turn, NPCs who are werewolves themselves actually attack you if they see you turning.

the game code is so confusing that if you transform in front of the companions they freak out.

I agree. characters that possess the power of the beast should use it. Aela should... maybe not Farkas and Vilkas since they seem to wish to be cured.

Thankfully there are mods for PC players to correct this oversight. But I agree, there shouldn't need to be a mod for this.