Having reliable hard limits for production accounts can be technically difficult as you need to do billing in real-time and also make decisions on what services to kill once the limit is reached. Do you just stop VMs, do you automatically delete data from storage. Many things could result in loss of production data.

There can be also many reasons for the budget overrun. It's not always a user error. It could be issue with the platform itself such as error in billing system or faulty autoscale logic. Or it could be caused by an external event, such as denial-of-service attack.

(Not sure how things work with the MSDN subscription credit, but at least you are not supposed to be running production workloads with those)