I don't think there's even a limit. The limit is a soft limit enforced through the UX of the tool, the features it provides, or even how it's marketed. There are always going to be high cost users and low cost users, service providers know this and build it into their revenue modelling.

Another example is home internet connections. They're unmetered where I live, but I'm also told I can't run public internet services on it. Why? Because with "personal/home usage" there's just a practical limit to how much I can use my ~1Gbps pipe, whereas if I ran a public service I might max out that pipe. I'm a pretty heavy user (~60GB a day), but that's a world of difference from the >10TB I could theoretically hit.

> but I really don't get why people are in a hurry to ruin a good thing

This is the crux of it. I like services limited by practicality because they're a heck of a lot cheaper. If people want more usage there's always API billing, they just have to pay for what they're actually using.