Political parties, and presumably the official 'in' and 'out' campaign groups, are entitled to use the full version of the electoral register for electoral purposes. This includes using the electoral register as a junk mail list. (Strictly speaking, there's no definition of what are and aren't electoral purposes but our political masters agree that it's absolutely fine for them to send you junk mail. So really, the reason is that's there's one rule for them, and one rule for us.)