I don't get it.
When you open invites like this, you're looking for new blood. You may have certain requirements, applications, etc. but in the end, you're trying to receive some new, active members that could contribute to the tracker.
If you want to limit site recruiting purely to user invites in order to stay low, so be it. But initiatives such as these are meant to be publicly shown in order to attract users that otherwise wouldn't be able to obtain invites through normal means, ie user invites. Where else would these things be posted, other than a filesharing oriented site such as FST? A blog? Another forum? Unless I'm sorely mistaken and there's some magical, secretive form of communication that's completely invisible to traders (or even if there is something remotely similar, it would have to be so "secretive" that most of the users there probably already know eachother, and are already on the site), this response just seems completely hypocritical. Anywhere that you could post this announcement to potentially allow multiple users who don't have a better method of obtaining the invite, will come at the risk of potentially inviting traders.
It isn't possible to avoid that.