The issue here is representation -- not merely the right to agitate for reform. The argument, as given, is that by going abroad, and placing oneself at a remove from the democratic process one has given up the right to political representation. That's because it's not entirely clear how the other reasons are supposed to work. Paying taxes can't be a necessary condition of political representation, since tax-evaders; all those whose earnings fall below the minimum taxable amount; the churches and charities which take direct tax exemptions; members of religious orders who take vows of poverty and the MPs who so famously managed to protect their incomes from the taxman, all retain their right to participate and representation. Presence in the country for voting purposes simply folds into the first point about living abroad, since what matters here is the effect of location on one's right to political representation.
Read the rest of his argument here.
