I've always wondered about this. The subscription model is so ingrained nowadays that I cannot think of any other web service of any kind which uses a one-time fee.
Ours charges monthly, but the current month is the limit of any normal customer’s commitment. The first time we went out to an exhibition and met some prospective customers face-to-face, a disturbing proportion of them had assumed that because we charge monthly, we must have some minimum lock-in period of a year or more, like their mobile phone plan or broadband contract.
This expectation that they had missed the long term lock-in somewhere in the small print was clearly costing us potential subscribers, even though our site simply said the price was X/month. If memory serves, we converted at least a couple of people on the spot as soon as we confirmed that this was not the case.