Something I haven't seen mentioned (at least, not given the importance it might deserve)...
Whistler does not own the vocoder. Whistler pays DVSI for the use of their vocoder.
It's conceivable (read: likely) that GRE and Whistler paid DVSI some amount per unit for P25 decoding - that would cover the PSR-800, PRO-18, and PRO-668. It would also cover the first revs of the WS1080/88.
Now Whistler says "we'd like to support DMR on the WS1080/88". DVSI says "well, you're going to have to pay us an additional $X for each such unit". Whistler says "that's fine, here's how many WS1080/88 units we've sold".
Whistler releases DMR code for the WS1080/88, users download it, DVSI gets paid, and everyone's happy.
December 2016: anyone with a PSR-800, PRO-18, or PRO-668 can now decode DMR. DVSI gives Whistler a call and says "Hey, you know all of those scanners going back to the PSR-800? Yeah, well, we're going to need that same $X for all of those units." And, pursuant to Whistler's contract with DVSI, Whistler has to submit an accounting of every unit imported into the USA - whether made by GRE or Whistler - and cut DVSI a big fat check.
Radio Shack isn't paying for that.
Eric Cottrell isn't paying for that.
All the PSR-800, PRO-18, and PRO-668 users aren't paying for that.
Who does that leave?