Partly this is due to the way we do the test in the UK. I can quite see that someone who takes a test on a 125 or 250 and then buys an FZ6 could quickly get out of their depth. But the way we do it in the UK is on a Direct Access Course on 500cc bikes - normally an ER5 or CB500. You spend around 5 or 6 sessions on the bike, each session lasting between 6 to 8 hours. At the end you take a 2 part test which is not easy by any means. The training is quite intense and as a result, by the time you pass your test you are going to have acquired reasonable bike handling skills, albeit at a basic level.
My FZ6 is my first bike, full stop. I did a DAS course over 5 days - from nothing to full licence in a week. I quite take the point that it's a whole new skill from car driving, but 18 years' driving experience and the IAM advanced driving qualification have given me good road sense in general terms. I've found the Fazer to be a steep learning curve, but one that's certainly manageable if you're sensible. I wouldn't want to be on a Gixxer, R6 or similar with such limited experience, but the FZ6 has been great, and is a joy to ride.
So if anyone's done the DAS but is scared off by some of the warnings above, I'd say don't disregard the FZ6. Yes, it's certainly a powerful bike, but if you are aware of your limitations and treat it with respect it can make a very good first big bike.