You've touched on two things. One is the harm principle sort of thing, which, you're right, absolves the government of the harm principle, and it says that governments meet the test, and in fact are positive, so that's a check mark. But there's also the financial implications and the blurring of lines between what will be discussed in a political campaign and what should be spent by political parties as opposed to governments.
Political parties in the campaign are in the business of persuading people to vote for them. Why should governments do that on a party's behalf?