Yeah, I would suspect this is intended. The idea being the kids (or parents, really, in communication with teachers) who opt-in to Algebra are more likely to have good study habits, etc. Kids tend to prioritize what the other kids around them prioritize, particularly if there aren't enticing alternatives, so other than raw aptitude the biggest key to success in academia and elsewhere is being surrounded by others invested in the same pursuit.

It's the same rationale more liberal localities use to hold back academically strong students and keep them in classrooms with everyone else. Except you need a critical mass of engaged students and an environment where the less-engaged students are less likely to self-segregate and stick to themselves. I think this is why the liberal policy has roundly failed to achieve the outcomes studies promised. But for the same reason, I would think the risk to the studious kids of adding a minority of bright kids with poor study habits would be minimal. OTOH, the academically successful cohort succeeds precisely because their parents segregate them into higher performing environments; they're not thinking quantitatively or care about averaged group outcomes. What they're doing works for them, so they're gonna fight back tooth-and-nail.

There are parallels here with the rationale many used to justify racial segregation, and that stills echoes today in terms of the distribution of parents who understand how the system works. But by-and-large I think what undergirds the parental hand-wringing and pushback today are more direct heuristics--the failure to choose to opt into Algebra, etc, communicates unsuitability for the higher socio-economic class.