I think this sums it up pretty well. It’s much less expensive to increase the number of students by more effectively utilizing existing resources than it is to build new infrastructure to accommodate the students on a Fall/Spring system.
You may have a point, although I suspect that AP and DE credit is already baked into the policy. If we assume that the average FL student carries 6-9 AP/IB/DE credits that can be applied towards their degree, then the 9 credit summer requirement means that the student is off-campus one full Fall or Spring semester. Resources that could have been used for that student can be applied to another student, increasing the overall number of students that benefit while minimizing the incremental cost.
Overall, I think FL is doing a great job in making sure that HS students have access to free AP, IB, and DE courses. If the price for this is a 9-credit Summer, online, or study abroad requirement, then I’m willing to pay it. From what I can tell, ~200,000 students graduate HS each year, and the number is growing. If we want to make 4-year degrees available to any student that has the desire and aptitude to earn them, then the state needs to carefully manage its resources to keep it affordable.