Walkers on the MMP's SHOULD have just as much right to walk on the pavement (single file, not taking up more than half of the trail width), but all too many carts don't want to slow down to let opposing carts go by before going around the walkers...so stepping off in the grass is simply a prudent act of self-preservation.
Walking facing traffic is obviously the smart thing to do, but there might be an argument made in walking in the same direction, i.e.:...at least then you'll never see what hits you.
