To be fair, there may be other reasons behind this than Abema's promotions (which, like I said before, are weird, but don't entirely make sense as reasons for the region lock). It could be down to the licensing for one/more of the songs on the setlist (i.e., not being approved for release to some other countries--lots of songs sold digitally in Japan aren't available in the US, for example), or even a mistake in the configuration when the show was added to Abema.
I just hate the idea of a region lock on a concert, though. Miku is the world's diva for a reason, and I passionately believe these sorts of things should reflect that.