[1] http://arstechnica.com/information-technology/2015/06/micros...
How exactly does one freely and easily add (reasonably expected) functionality to OS X? Due to how much escalation was required, even Apple themselves thought it should work.
My point being, unless Apple sabotaged the hardware to make it only usable with their brand of displays, there's really no harm done here. You don't buy a 4K TV, hook it up to a 2005 era DVD player, and expect to see 4K resolution on the screen just because your TV had a plug that accepted the DVD player's cable. Just because the plug fits doesn't mean the capability is there. If you want MST over DP, use the software that allows for it, and if that means installing another OS, then so be it. Petition Apple to allow MST over DP as well as Thunderbolt and explain to them that they lost a Mac user to Windows or Linux because of this.
I hope that makes it easier to understand.
There are no Apple displays that support daisychaining via Displayport either, so this whole notion that Apple actively sabotages 3rd party DP displays is nonsense.
Now, one might argue that you would want them support this because it's technically feasible with the current hardware (I have no idea), but there might be a ton of reasons why Apple doesn't.
AFAIK, such displays weren't even available when this generation of Macbooks were built, so the most likely explanation is the Apple simply didn't bother initially and until very recently had no reason to revisit that decision.