This rule was written because Doug Henry was taking intravenous fluids between motos and winning the second motos. To stop this activity from spreading through the pits, where riders might try to administer the fluids by themselves, the AMA banned the practice for the safety of the riders. That doesn’t mean it stopped, but only that it had to be done in secrecy (in the riders motor homes).
The most remembered utilization of this rule, even though it wasn’t by the AMA, was at the 2015 Thailand GP were the Grand Prix riders began passing out from the heat in moto one. They were given intravenous fluids by the doctor, but told that they could not start the second moto. Jodi Tixier, who was the 250 World Champion, insisted that he should be allowed to race — and when told that he couldn’t, Jordi physically accosted the track personnel. For those action, Tixier was fined $5300 and put on a one month suspension, which meant that would have to sit out the Argentinian Grand Prix. It ended his chance of defending his 250 crown—which went to Tim Gajser in 2015.