Plenty of recent Disney films have had 3D amaray versions in the UK (Tangled, Brave, Finding Nemo, Lion King, Monsters Inc and Monsters University is forthcoming) so how is Disney responsible for this steelbook not having 3D?
If Zavvi are doing an exclusive steelbook then I'm sure they could choose between having 3D copies included or not. Case in point is Monsters University which is getting a 3D version in the UK - so why does the Zavvi exclusive not have 3D? Claiming Disney are somehow withholding the disc seems unrealistic when it is freely available in the UK in other versions.
You only have to look at the fact that smaller sites worldwide can get lovely 3D steelbooks signed off by Disney to see that, with some effort by the retailer, all extras and features can be included.
End of the day it is cheaper for Zavvi not to include an additional disc, that is obvious, so they can have a high price with minimum outlay to get maximum profit. That's their call and people are free not to pay if they care that much.
I personally don't care for 3D, so it doesn't bother me either way. This film is one of Disney's, ahem, misfires - so I'll be giving it a wide berth.