in an adversarial system like we have in this country and they have in most of the states
then the polices job is to secure convictions, they are not at all interested in if that happens to be fair or not.
do they think they have enough evidence ? yes, then you will be charged
if you have an arguable legal defence, then you will have to argue it in court, the police are seldom interested in shades of grey, consequently you need to be very very careful what you say,' loose lips cost ships''
such advice as they do give tends to be on the simplistic side and completely avoid, suggesting that there are shades of grey or just ignore statutory defences all together, they are not going to help you to get off or encourage you to argue the toss with the police on the ground, who are telling you you must or can not do something, based on info provided by a police web site, wheres the sense in that ? they just want people to do as they are told regardless of if they have a legal obligation or not