I think the main reason the limitations were imposed was to stop people spamming New Members, Off Topic and Classifieds with endless "welcome", spam and "glws" posts respectively just to reach 60 posts to sell something.
This is a keyboard forum, and the ideal would be to encourage people to post about keyboards and related items.
A lot of people see the post count as important, although it can be misleading as someone with a small post count can have contributed hugely to the forum, and others with a large post count might be total noobs.
By not including posts in those three areas in particular, it encourages otherwise casual members to become more involved in the community.