In my humble opinion, the only reason for something like this to be found, and also as some people stated along these pages is - stupidity.
Even though immersed into a completely virtual, parallel environment in which you can barely have any certainty about what lies behind someone's screen, there are those who wish, or think it's better to blind or fool themselves under a meaningless statement of 'voice/cam/sex-verified', and some clubs (unfortunately) prefer, instead of taking -AT LEAST- some time into investing in -QUALITY- of its dancers/escorts, catering to the minimal expectations of patrons who just want to jerk their minds with what they want to believe in.
That's all. The same goes for those clubs which base their applications on 'SL Age' of the residents. It -DOESN'T- matter if one has a large and well-developed experience with complex, top-quality RP, if one emotes marvelously-well and do its best to draw the attention and interest, to the perfect extent, of a client, if one has the skill it takes to make them loyal and is willing to join such place, still having it acknowledged by others. It just -DOESN'T-, if this same one doesn't meet the required SL age.
If you can't voice-verify, even if making use of voice-morphers, or if you're a month from being as old, in SL time, as expected, -THEN-, the average baboon who's been around for twice as much time in SL, emote in an extremely poor, ridiculous way, can't get past the average '..H elo (name) welcommm, xXx KISS - KISS xXx.. )'' and spend like 10 minutes spamming awful-made posts about how she feels like blasting her ovaries/his shaft against the pole after being tipped, while completely stepping and ignoring any minimal grammatical accordance, if not copy-pasting the same it was done before, is - BETTER- and most-suited than the first applicant commented about, when being able to voice-verify through countless ways, being a manager's friend or whatever.
Overall, minimalism and lack of consideration about -ANYTHING- other than the number of gathered clients, even if a club has 40% of semi-demented individuals among its ranks (and matches the personality of the average wank'n'clicker), are just way better and more interesting for a place than investing a bit on quality and confidence on a good team of dancers/escorts, and so, who can't make use of endless manners, accurate or not, of gender-verification.
Of course, this is -SOLELY- based on my personal opinion, and on what I could find out among clubs, dancers, managers, and so on, 'till now, and this idiotic policy adopted by some (not on its majority, thankfully, apparently). I just don't like the -mandatory- tone with which some clubs come to require it.