Exeter are not alone in securing ball at the breakdown by going off heir feet, they're just better coached at doing it. The first player or two arriving at the breakdown will go straight off their feet and then to make it look vaguely legal adopt a sort of press-up position supporting their weight with their arms, they are still clearly off their feet but it's supposed to fool referees into making it look like they attempted to stay upright.
Then you've got those "clearing out" once opposition players have arrived. Clearing out is illegal, it breaks several laws; Law 15.5 "An arriving player must be on their feet". Law 15.7 "A player must bind onto a team-mate or an opposition player. The bind must precede or be simultaneous with contact with any other part of the body." Flying off your feet at an opposition player fails to meet both of these laws so should be penalised quite apart from Law 9.11. "Players must not do anything that is reckless or dangerous to others".
One of the basic principals of the game is that it is played by players on their feet, once you're off your feet you're out of the game. I appreciate that at lower levels of amateur rugby it's not going to happen but are you telling me that supremely fit professionals are unable to stay on their feet? If they were penalised every time they'd soon stop. The big problem for referees is that you don't penalise every offence only those that have a material effect; if a plater is tackled and two of his team mates dive off their feet but there are no opposition players challenging to the ball they've gained no advantage, no material effect, play on. If Jack Willis is quick to the ball and in his trade mark "Jackling" position it should be a penalty. Ping players every time for going straight off their feet and they'll stop doing it and breakdowns will become a far clearer, fair challenge for the ball which is that they should be. Far safer and easier to referee into the bargain. It will mean a further shift in playing styles and coaching and won't suit every team but it would make for a very exciting, fast flowing game. Speed, strength and technique gets rewarded whether you're in attack or defence. That's what rugby should be all about, at the moment it's still "How can we bend the laws as much as possible to get an advantage"