Conveniently, DVD players sold in PAL regions would often play NTSC too, although not the reverse.

I think decoding NTSC is a subset of what's required to decode PAL, if I remember my electronics classes well enough.

Or, searching around, it might be because region 2 DVD players had to support PAL, NTSC, 50Hz and 60Hz. Region 1 is only 60Hz NTSC.