It doesn't support it, exactly, it supports a compressed image in each of the multiple streams transported, which can introduce quality problems when doing the production work people use higher end Macs for.

"the combined data rate requirements of all the displays cannot exceed the limits of a single DP port" -- https://en.wikipedia.org/wiki/DisplayPort#Multi-Stream_Trans...

Arguably, compared to TB ports transmitting a full quality image to each screen, this could be "unexpected behavior".