Terje Hermanseter (6/6/2016)
Since we don't have a 2016 server that I could test this on, I tried to read the documentation for clues. Found this article, https://msdn.microsoft.com/en-us/library/dn246937.aspx, but sadly it didn't say anything about JSON. So I chose the wrong answer 😛
Yeah, I couldn't find any documentation for this and don't have a handy 2016 instance, so I also picked the wrong one--I thought the question might be asking about the general inability to alter the table schema of a memory-optimised table, for which constraints are an exception. Learned something, though, which is always good.
I suppose the obvious question is, though: WHY is this limitation there?