Safe = adequate. One must ride ‘adequately’ to be ‘safe’ in eventing, even if only doing it for fun. A fall can be life-changing at the lowest levels too. The USEA has a duty to ensure competitors are as safe as reasonably possible. A good deal of statistical evidence now indicates where that cut off point is between ‘likely to do ok’ and ‘unlikely to do ok’.
So only as an example, (I can’t look up USEA data since it isn’t published, unlike BE) the stats might indicate 45 in dressage and 96% subsequently jump round without falling but 50 in dressage and 73% jump without falling. So make 45 the minimum score achieved to proceed in safely. Numbers are fairly unambiguous and seem objective. Imagine if we all had to pass a riding test in front of trained judges before being allowed to proceed … Oh, yes, dressage. But like any boundary, where it is placed and what happens to individuals close to it but on the wrong side will always cause debate and unhappiness. Tax levels, voting district boundaries, school exams…
The other thing is that such boundaries are not fixed. Times past, a 40 in dressage at Badminton was deemed quite respectable. Today, more than 30 probably puts you out of contention and sub-20 is getting more common. Riders have worked very hard to improve dressage, not least because they have found it helps performance overall.