Weight of the implement is not the issue. Unlike a snow blower, plowing is usually done at speed to make use if the momentum of the tractor/operator. When impacting an immovable object at speed, the force is transmitted up to the frame from the cutting edge. Yes, spring loaded tilt mechanisms help, but they need space for the blade to actually tilt. With a wall of heavy snow buiit up in front of the blade while plowing, that space is somewhat restricted.
Also, it takes axle torque to push a blade, considerably more so than mowing a lawn. To achieve higher torque numbers requires weight on the rear wheels of the tractor, usually in the form of liquid ballast, wheel weights, tire chains, and weight added to the rear of the frame.
Once weight has improved traction to approach the maximum torque capability of something like a K46 with its 171 lb-ft axle torque rating, thoughts must go to where on the tractor that torque is being applied, to wit, the axle mounts to the frame. ANY flex in the frame or looseness of the bolts will result in damage in the long term. Just as the higher pressure generated in the hydro will contribute to excessive wear in the hydro.
While you obviously disagree with the dealer, the reality is that he is bang on correct. As with anything applying a load, damage doesn't (usually) happen with the first application. Repetative applications are another matter entirely. See the comments above regarding axle torque.
When I was 10 years old, my dad bought me a 30" magnesium scraper to shovel the 3 driveways that I had contracted to keep clear. I used that scaper for 9 years and the first pass through 8" of snow was always a killer. In my off hours, I skated on the school rink where it was the job of all the guys on the rink at 7 and 10 o'clock to use the 48" steel scrapers to clear the snow built up from all the blades carving the ice. Skates on ice have great traction. Pushing a full load of snow with a 48" blade will wear out legs that have been toughened by 2 months of several hours of skating every week and 12 - 20 (X3) driveway clearings.
When dealing with loads on an LT hydro, if you can't do it for a short time by hand, it's going to have a negative effect on the hydro in the long term.
