I don't know if anyone has seen or heard about the review videos that Mark Andrews does on YouTube, but he reviewed a play like this and I agreed with his assessment (disclaimer: can't find the video yet, so this is all off my recollection and might not be what he actually said). Basically he said that tripping the ball carrier in and of itself is completely legal, but how it is done can escalate it to a UNR or similar type of foul. Now adding my assessment of that...it's a judgment call, but in my opinion if a defender just sticks his legs out or even comes sliding in from the side and gets his leg in front of the runner then we're probably clean. If the defender comes in with a foot up (or both up) and contacts the ball carrier with the spikes of his cleats or if he takes the runner out from behind, making major contact in the back of the legs, then we have a very dangerous play that would be supported as a UNR.
I don't feel that this would qualify as "making up rules" as we have a lot of plays that are legal unless done in a dangerous manner, but I'm curious what you all think of that interpretation.