I don't think that only allowing the purchase of tether to retail really meets their original plan.
The whole point was meant to be that anyone could take their coins and go to tether and get exactly 1 USD for each one. It was designed to solve the problems people were having cashing out from exchanges. The idea was that you sold your bitcoin on poleniex or wherever for USDT the then took that to the tether website and got your USD. Safe in the knowledge that your USDT would be exchanged exactly 1-1, with no bid/offer spread or moving price. The reason this was needed was because most exchanges were really struggling to get their banks to allow the USD processing.
In reality, you could never really do that. And now exchanges have been able to get the USD payments working, so the USDT<->USD exchange market has picked up the slack and now serves the purpose that the tether website never realised. Although it's now not quite 1-1, there is a market and a spread and prices move.
As others have mentioned, the likely reason they never have is that they would hit exactly the same challenge all the exchanges did with their bank and the KYC rules.