If you have an elective health insurance system where people choose whether or not and when to participate, it implies this restriction.

Say you have private health insurance companies but don't allow them to prohibit members based on pre-existing conditions. Everyone will simply not get health insurance to avoid paying the premiums until the day they need it. That would make health insurance companies completely unviable financially.

This is why Obamacare both eliminated the pre-existing condition restriction and mandated people get health coverage. You can't have one without the other or you get the free-rider problem.

This is another reason why healthcare should just be universal. It doesn't work at all like an actual efficient market product.