Imagine if dereferencing a null pointer in C was defined to segfault. Currently, it's UB, which means the optimizer assumes it's not null, and when you break that assumption, weird things happen, the least scary of which is a segfault.

This is similar. "program error" means "You're not supposed to do this. But the compiler will not assume that you've not done this; it has your back. If you do this your program will exhibit defined behavior. Perhaps undesired behavior (panicking, or segfaulting), but the behavior will be defined."