I think the editors of C99 and POSIX would be surprised by this too. Any non-buggy fread or fwrite should retry on EINTR.
But maybe not unconditionally retry. Say you got SIGTERM while waiting on network I/O and need to go flush something to disk and exit (instead of just resuming your wait or calling arbitrary code from your signal handler), or you got SIGCHLD on a subprocess and need to clean up stuff related to that.
Actually the introduction of this in the kernel back in '97 or so caused the company I was working for a lot of grief, and a quick recompile, since after a kernel upgrade the binaries we supplied were no longer working reliably.
(NOTE: In this context, "PC" means "Program Counter".)
"... The PC loser-ing problem occurs when a user program invokes a system routine to perform a lengthy operation that might have significant state, such as IO buffers. If an interrupt occurs during the operation, the state of the user program must be saved. Because the invocation of the system routine is usually a single instruction, the PC of the user program does not adequately capture the state of the process. The system routine must either back out or press forward. The right thing is to back out and restore the user program PC to the instruction that invoked the system routine so that resumption of the user program after the interrupt, for example, re-enters the system routine. ..."