An army in which individuals voluntarily contribute money....

"First, that sort of army is a private army: if the money is raised privately why should it be given to a government army?"

Cyrus Ferdowsi

Alan seemed to imply that he had to choose between two evils. He implies that a military, funded by taxation, is wrong because people are forced to contribute, even if they do not want to. He says he would like to use "subscriptions" to create a private army, but an existing government would then use taxation to stop him. So either way the government is preventing him from doing what he wants. So if he has to pay taxes to the government, he at least wants the money to go to fighting tyrants and not towards keeping him in prison.

I am pointing out that Alan's grim choices are not so limited. If he could raise substantial sums of money (say 40% of the military's budget per year), and promise it to the military in exchange for the government cutting taxes plus some input into how the military is utilized, he could make progress towards having a military funded by voluntary contributions. In order to raise that much money, contributor's opinions about the role of the military would surely need to be taken into account.

Alan may not be able to have a private army, right now, but if he could raise nearly half the money needed to run a military in a year and exchange it for lower taxes, he could make substantial progress towards privatizing the military.

"Perhaps the people you are talking about feel they can get a free ride of security on other people's private donations?"

Yup. You got it. The real reason Alan is not participating in a process right now that would lead towards privatizing the military is that it wouldn't work. He can't raise that much money for a military because of the "free ride(r)" problem.