Why would boots put any more stress on your knees?
It's a good question and one I think we have to answer empirically. aaron hadlow was injury free for 12 years never got an acl tear until he went to boots got one that same year. Kevin langeree got his acl injury in straps in 2010. youri zoon got his acl torn twice once in 2007 and once in 2008 in straps, as far as I know alex paster never got an acl tear and he rides boots. ruben lenten had the worst injury of his career in straps and has still not recovered completely with a broken ankle. Bruna Kajiya switched to boots and tore her acl. I watched a friend of mine tear his acl. He had been riding in straps for about 10 years, and occasionally switched to boots to unhook. It looked like absolutely nothing, landed a bit awkward, and the next thing I see is him body dragging back to shore.
I think there's a slight trend. Boots seems to be a bit worse for the acl. Straps seem to be a bit worse for the ankle.
I have no idea of the physics here just watching the pros and the local riders and adding up the injuries. Because I agree with you based on the physics it seems like boots would be safer for your knee but it doesn't seem to be in real life.
I might change my mind about this, wish I could compile a better list of the pkra riders and when they switched to boots and their injuries. You also have to add up the years injury free vs injury free in boots.