Nobody here knows exactly what happened and neither do I, but I suspect that the PC's shoulders were not broad enough to resist the 'pressure' of the allegation and that he/she therefore felt the need to investigate and ask for a breathsample.
The crew's knowledge of the law may not have been enough, their shoulders not broad enough nor the situation conducive to a refusal, see FL's excellent advice on this.
It is all well and good for all of us here to be very clever about the ins and outs, and who should have done what, and what we would have done in the same situation.
This occurence is a reflection of what our society is all about, and a pretty sad state of affairs it is too.
Make no mistake, society, that is you too.