One day a big light bulb may illuminate over a prossies head..
"These blokes who phone me are prospective customers, not timewasters. It is my job to turn them from prospectives into real business!"
Then the next bit of illumination is that most blokes need a camouflage activity to hide their punting activities, such as a work trip, gone fishing, etc. If the camouflage activity fails, (eg work trip cancelled, inclement weather for fishing) then the visit to the prossie fails too. Admittedly most blokes are decent enough to cancel as soon as they know. If the prossie doesn't accept this is a normal part of her job, and gives the customer aggro, then she will never see that guy. On the other had, if she simply accepts it, then she might see the guy at some point in the future.
Some possibility of money is better than zero possibility!
I suppose any prossie too stupid to understand this will soon run out of prospective customers in her area!
Personally, I explain at time of booking "This depends on a work trip, so there is a risk of cancellation.. Feel free to take a 'definite' booking instead if one comes along". I've never had any grief off a prossie when having to cancel... "Yes, I did say there was a risk, remember!"