Lots of democracies have legal definitions for what counts as political advertising. Obviously Twitter doesn't have to use those definitions, but there are fairly clear guidelines in a number of jurisdictions. And it doesn't have to be a situation where they set rules for life: taking a firm apolitical stance on advertising likely requires reassessing the rules over time. But it is doable.

What will be really interesting is whether Twitter tries to work with governments or whether they unilaterally come to their own rules. There be dragons on both fronts.