In the report of the final EWC round, it was written that DK wanted to ride a 300, but it was vetoed by KTM.
Perhaps KTM has made promises to the OEM's about not letting too many 2S trokes to participate..?
Of course they have to keep their 4 strokes popular in order to keep sales going.
But sad to hear that the riders are beiing forced like this.. 
In No way would I think that had Anything to do with Any promises to Any other manufacturer. Why in the hell would they Need to make promises to other mfrs to not promote / race 2ts in any series other than AMA Nationals / FIM MX World Championships?
Perhaps you are thinking of the time a couple of years ago where it was reported that KTM was willing to promise to the Japanese manufacturers that they would not put any of their contracted riders onto a 250SX if the AMA were to allow capacity parity?
I think KTMs 2t sales are the main reason they stay afloat and have the money to sink into more 4ts and their road bikes. The fact that other mfrs have bailed from 2ts is THE biggest windfall / gift horse that any manufacturer has ever been delivered.
I think there is way to much silliness with regards to conspiracy theories amongst my fellow 2t enthusiasts - And they too often are latched onto by others to criticize and ridicule the 2t side of things.
It's entirely understandable on KTMs part, to say, no DK, ride the 4t. It is was what he was signed to ride in the EWCs, and it's what they are desperate to sell.
KTM sell every single 300 they make, and I daresay they could sell at least 25% more if they chose to up production. It's the 4ts that they (and other manufacturers ) continually have backed up in the warehouses, and they (most mfrs) nearly always have to bring out major deals / cash back schemes to try to reduce stocks.
