The usage limit on your $20/month subscription is not $20 of API tokens (if it was, why subscribe?). Its much much higher, and you can hit the equivalent of $20 of API usage in a few days.
API limits are infinite but you'd blow through $20 of usage in a maybe 1 hours or less of intense Opus use.
The subscription at $20/mo (or $200) allows for vastly more queries than $20 would buy you via API but you are constrained by hourly/weekly limits.
The $20/mo sub user will take a lot longer to complete a high token count task (due to start/stop) BUT they will cap their costs.
I dont like it either, but its not an unreasonable restriction.
Fix the limits, so the limits are reached at a rate that sustains their business.. ? obviously this WILL happen eventually when they need to pay for things.
Their bet is that most people will not fill up 100% of their weekly usage for 4 consecutive weeks of their monthly plan, because they are humans and the limits impede long running tasks during working hours.