Hi AOG,
That is quite odd. Giving space to BMX's is always a good idea as they'll often be without brakes (so they can spin the handles around during tricks). I wouldn't worry about them too much though, they're usually being controlled by those who heal quickly!
Racers on the other hand are generally pretty tooled up and, to be honest with you I only ride standing up when moving off from the lights (i.e. sprinting) or climbing steep hills. This is the kind of thing I mean by racer:
http://www.bassouk.com/ (I only say this because I find it odd for the bikes to be lumped with BMX's and thought there may be a chance of a misunderstanding),
Interesting you associate the "standing up" with dangerous though. I find I give more room to the couple heading to the pub on a sunny afternoon (their first bike ride of the year) on broken down "mountain bike" style bikes - they always seem the most dangerously unable!
All the best,
Spare Ed