Honestly, for a huge set like the Alien Anthology, it'd be cheaper for them to have one master, and distribute it to all regions. They don't have to worry about PAL/NTSC differences, either, and a LOT of Blu-Rays are region-free (I've seen infinitely more region-free BRs than DVDs).
I would be incredibly surprised if the US Blu-Rays look any different from the European Blu-Rays.