I suspect they could have easily found a more FOSS-friendly license which achieves their goals. The exact license depending on the exact goals, but presuming the "normal" GNU GPL isn't appropriate, the GNU LGPL or BSD/MIT licenses might be possibilities.
The Ms-PL, on the other hand, seems to be a license which is gratuitously incompatible with the GPL, simply to be incompatible.
I've seen projects where the README essentially stated "We hate the FSF, so we're using this roughly equivalent and yet incompatible copyleft license to piss them off," but I don't think Pixar are jerks, so I'll put this choice down to simple ignorance.
The Ms-PL, on the other hand, seems to be a license which is gratuitously incompatible with the GPL, simply to be incompatible.
I've seen projects where the README essentially stated "We hate the FSF, so we're using this roughly equivalent and yet incompatible copyleft license to piss them off," but I don't think Pixar are jerks, so I'll put this choice down to simple ignorance.