It makes sense when you consider why we have it:
First, it gives us a gauge in which to see a user's attitude, posting behaviour, and understanding of the rules and atmosphere of the forum. It also allows us to see someone's long term activity as a regular contributor to the site, to avoid someone who just shows up periodically and blasts a bunch of posts before disappearing for a month. It provides us with a way to separate out people who are rude to other users verses the users who are helpful, who is eloquent and a good communicator or who is incoherent. It is super important for us to see how potential moderators interact with other members of the community. While this isn't a perfect metric for judging someone, it's really the only reasonable one we can use. We can't do any of that with someone who barely posts.
Second, we want moderators who are actively involved in the site and the community. Sure, you don't need posts to strictly moderate but being a mod here is about more than just infractions and bans. We want mods who are credible within the community and who are capable of guiding discussion and leading by example. We want regular posters to be familiar with who the moderator team is and don't want to surprise the boards with people they've never seen post suddenly being in charge. All of this requires them to be an active poster. Reading is great, opting not to post if you don't have anything worthwhile to contribute is great, but there's no way for the mod team or the users reading the board to differentiate between someone who is doing that and someone who is just never around.
Given all that, I think the post count requirements are reasonable (most average out to less than one post a day which is basically nothing) and we have adjusted many of them over time to fit the activity level for that forum; things like class forums will always require less posts than, say, GenOT.