Has anyone else had this happen? Is it within the scope of their warranty policy to deny warranty if you didn't buy it from the truck, or is it a rogue salesman here or there being uncooperative?
The simple answer is yes, yes and yes. They state in their warranty info:
"Snap-on warrants to Customers who purchase Product from authorized Snap-on distribution channels for use in their profession that Snap-on® brand Products and Blue-Point® brand Products will be free from defects in workmanship and materials for a duration described in each Product's warranty code."
"The following information will be required with the customer’s warranty request: (1) date and proof of purchase, (2) where customer purchased the product, (3) full name, (4) shipping address, (5) phone number, (6) e-mail address, (7) item number(s) or approximate weight of return package. Warranty requests that do not include all of the required information will not be processed."
http://www1.snapon.com/display/termsofsale.nws
Snap On still makes great products, and if you have/maintain a decent relationship with your rep. you won't have any problems.