When you work for somebody else (or with somebody else) then you try to do as good of a job as possible, but the ultimate responsibility still lies elsewhere - might be your boss or could even be the group. There will be other people who interact with your code and might spot errors. There will be people who are trained, in some capacity, to figure out ways to mitigate against accidentally generating very large bills. It is exceedingly unlikely that these points hold true for a solo developer working on a main project, let alone a side project.

Even if you are hyper competent and can probably get all of this correctly, you can't rest easy. You simply don't know whether you did everything correctly or not. Just one dumb mistake can saddle you with an enormous bill.

This is just like gun safety: don't point your gun at anything you're not intending to shoot. Mistakes happen and the consequences of it can be catastrophic.