I don't see why it is "alarming" that not everyone followed you through the door?!
If you're hungry, you're hungry and will quote for the work. I don't see it being rude, at the end of the day when visiting a customer or potential customer you're there to give a quote it is essentially tendering for work. What is the difference between giving a quote infront of other customers and tendering for work online?
I do see your point the customers always king, if you ring someone to come and price a job you should have the courtesy to tell them that there will be other plumbers there and you will want a bidding war for the best price ,this is not unreasonable to want the best price but who would turn up knowing that ? If they attend then they know what is required.it is unfair to waste people's time especially if they are self employed sole traders time is money to them.