When I read that, I immediately thought: that's incompatible with Pro/Rel.
This article has more details and agrees:
On the USL's bold plan to compete with MLS at the D1 level
www.usltactics.com
You need 75% of clubs in Div1 to meet the 1 million metro area population standard to comply. The article projects that USL would have 83% of the Division 1 teams in 1M population areas in the first year. But there's no way to ensure that is maintained if you have Pro/Rel. The Division 1 rules also require a minimum of 12 teams in the league in Year 1, and 14 by Year 3. So maybe they go to 16 to make my math easy.
75% of 16 is 12, and that would mean the only way to guarantee compliance year to year would be to have no more than 4 sub 1M locations in the entire USL system, because if you have 5 of say 60 teams in small markets, and all 5 end up in Div1 at the same time, you're non-compliant. The rule is basically designed to make Pro/Rel impossible. Maybe not by intent, but that's what it does in practice.
I kind of hate the Pro/Rel partisans because so many are obsessive and tiresome as they turn every conversation into a Pro/Rel discussion, but that's not right. If someone wants to make a go of an open Pro/Rel system in the US it should be possible. I understand you want benchmarks that make success more likely than not, and population is a useful benchmark in that regard. The rule should require a certain level of 1M population centers in the entire system, and then let it shake out however it does. The teams located in higher population areas have a presumptive advantage anyway.