Where did you see that he hasn't started throwing yet? I heard Buck says probably about 4 days ago the he started to throw already, though very short throws.
Besides, it's probably too late to DL him now. We have 3 games against the Nationals starting June 1st. Then there are 2 against the Mets on June 15 and 16.
If they DL him now it back tracks to May 24, he's he's going to miss all 3 games against the Nationals. Could be ready for Mets.
The other option is to shut him down and keep him on the roster, then maybe he will be ok enough for the Nationals.