Middle schools that do this do not make sense. Students who take algebra 1 before 9th grade should be presumed to be stronger-in-math students. So if there is any difference in algebra 1 offered in 8th grade or earlier, it should be in the opposite direction (honors or enhanced or some such).
The only reason the school districts may do this is that they may allow too many parents to push their kids more advanced in math than the kids really should be. Instead of saying “no”, or letting the kids struggle in algebra 1 taught the non-watered-down way, they degrade the educational experience for those kids who are appropriately in algebra 1 before 9th grade.