See
this on the conclusions on the Australian legislation. This
site links a lot of research published internationally, but a quick google will throw up hundreds of links, with lots of conflicting conclusions. An interesting line from the NCBI report states that cyclists and motor vehicle occupants have a similar risk of dying from head injury per hour. That would suggest anyone thinking cyclists should wear helmets should also support vehicle occupants doing likewise.
I haven't seen any research that would prove mandating helmets results in riskier behaviour. That would be very difficult to measure, and there are likely too many factors involved/ For example mandating helmets might discourage a disproportionate amount of conservative, risk-averse cyclists, and so the risk-tolerance levels of the remaining cyclist population would rise and individually, they are more likely to do something stupid like the behaviour or the cyclist in the OP. Bath University published a [broken link removed] finding that helmet wearing cyclists are more likely to be hit by passing traffic, so they suggest the wearing of a helmet also affects the behaviour of other road users around the cyclists.