It's not just the salesman, but the manufacturers too. I have a F150 Eco with both Max tow and HD payload. The manufacturer sez I can tow up to 11,300 lbs. ...but the receiver is only rated for 1,130 lbs. Thats 10% of the trailer weight. Ten percent tongue weight is on the edge of disaster!
It's almost impossible to find a trailer that has 10% tongue weight from the factory. The trailer manufacturers know that 10% is marginal and usually design around 13%.
The max I feel that I can safely tow with 1,100 lbs on the ball is about 8,000 lbs.
Blame the tow vehicle manufacturer as much as the salesman. In many cases, he's only regurgitating what the manufacturer publishes.