Yea because tyres are just going to sit there they aren't modifying the landscape, as was commented on with postcrete, or worse concrete, though of course someone will say they will leech out chemicals. Also they would be covered over by the structure once it is built, not like if they are just piled up at the gate and an eye sore.So, you're not wiling to remove rock and stones from a stream as it's not "environmentally considerate", but you are willing to to haul in a load of old tyres
One has to pick their battles.
I see that people will moan about any thing I mention online, just for the sake of it, so I just choose what I feel is the best idea and proceed.