The amount that couldn't be collected is less than 1% of the total generation. So my guess is it's due to decades old infrastructure where lines just happen to cross over to where the dividing line ended up. Something similar happened in Berlin, where two West Berlin train lines happened to pass through East Berlin briefly, and were allowed to continue to do so even after the wall went up: https://en.wikipedia.org/wiki/Berlin_U-Bahn

I can easily see the utility deciding that it's easier and cheaper to just let a small bit of power get stolen than incur the costs of changing that infrastructure, as well as the PR costs of cutting off that power. Overall grid losses are likely to be 5% or so anyway.