The Heart Attack Grill's "warning this burger might kill you" waiver is mostly just a meme gimmick. It's unlikely that eating a big greasy burger would cause your arteries to instantly clog and cause a heart attack within the time frame you're inside the restaurant.
Beyond that, liability waivers saying something like "we've taken all practical safety precautions possible for this activity and have medics on site in case of emergency, but there is still unavoidable risk that you must accept if you continue" are an entirely different thing from "We reserve the right to murder you at any time we please." which is in an illegal contract that can't be upheld.