Warner are strange.
The classic Time After Time with Malcolm McDowell remains unreleased over 4 years since it came out on R1.
Forbidden Planet has never come out on R2.
Same for Logans Run and Westworld.
And it does not appear to be anything to do with rights ownership either.
The O.C (teen drama) and Invasion all out on R2 with all the extras missing.
Its an old story.
Warner have released NTSC R2 discs in the UK before aswell so your theory would work.
Coincidentally,the majority of the Warner cartoon releases ,like Looney Tunes (loaRAB of extras missing on R2) and the tv stuff like Hong Kong Phooey,Top Cat etc are actually encoded for R1 and R2 and in some cases R4,but they don't get an official release outside R1.
Odd.
But with Warners track record,I'll pay extra for the R1 of this set