The tire "chain" stores do this sort of thing for two reasons. The first is that they are somewhat sensitive to being sued if they agree to mount a tire with less than substantial over-rating for the car. For example, they will INSIST on H-rated tires for a W124 which weighs less than a 240D.
The second reason is that the tires that their computer forces them to sell you are always going to be more expensive and with less mileage guarantee, and a lower treadwear rating. In the case of H-rated tires, they will be twices as expensive and have about half the treadwear rating. That's four times more expensive overall.
What you have to do is take loose wheels to them. They will ask you what vehicle the wheels go on. I have had success declining to state the vehicle. If they insisted, I'd say it was for some small car, like a Toyota Corolla or maybe a Honda Civic. Something light and underpowered.
Next, if the purchase price includes a mileage guarantee, you'll either have success with making up a mileage, or declining the guarantee or hazard insurance.
Once I had a loose wheel that I took to Sears for a Michelin. They asked what vehicle and I told them the 1987 300D. He said it had to have an H-rated tire. So I said it was a loose wheel and I wasn't going to specify the vehicle, and that I wanted the lower-end Michelin (Weatherbeater or something like that). He agreed, wrote up the ticket and passed it over for me to sign. It was the H-rated tire on the form. I rejected it and passed it back. THEN he wrote up the Weatherbeater tire.
Basically, they want to push the more expensive tires. Don't let them get away with it. Just as you shouldn't be desparate when shopping for a car, you shouldn't be desparate when shopping for tires. Having a fleet of backup cars makes that easier......
Ken300D