It's easy to convert gammas; the worst you get is banding when if you lose resolution and fa r movies this can be done offline, but when two color spaces don't align you'll have to truncate to the intersection where converting making the lager gammuts pointless. So given that nearly all big movies have been filmed and color corrected in P3 for years now, it makes sense to match their gamut, but match common display gammas, so that common sRGB content needs little conversion to be displayed.

OpenGL has native support for sRGB blending, so maybe keeping the same gamma allows them to use the same hardware to accelerate Display P3 blending.

Though I think sRGB's gamma is technically the much more complicated

       {  cs / 12.92,                 cs <= 0.04045
  cl = {
       {  ((cs + 0.055)/1.055)^2.4,   cs >  0.04045

and not exactly 2.2.

Thanks, I didn't know that. Do they actually use sRGB "gamma" (which has a linear region near 0) or do they use a straight 2.2 gamma like Adobe RGB does?

I didn't know that Adobe RGB has a straight 2.2 gamma. :)

I don't know the details about the Apple's gamma curve, but these guys have measured it. Perhaps it gives some answers. (I'm not sure how to read that graph..)

http://www.displaymate.com/Gamma_35.html