On the contrary, that makes a ton of sense - if you have a fuzzy system with a potential for people to trigger massive resource usage, it would be silly not to put a hard restriction on the tasks that are resource-intensive.
A hard restriction would be a regex or a simpler model checking your prompt for known or suspected bad prompts and refusing outright.
If NLP was that easy, we wouldn't have needed to invent transformer models, and we'd have had things as capable as ChatGPT about the same time that Microsoft was selling Encarta on CD.
The reality is, this soft fuzzy thing is the only practical way to minimise the Scunthorpe problem (and its equivalents for false negatives): https://en.wikipedia.org/wiki/Scunthorpe_problem