You can rm running executables on AIX 4 (AIX 3 is really old). You just can't cp over them or recompile over them. A common thing to do in AIX Makefiles is to rm them first before recompiling.

It's not just AIX, it's most unices. When a program is run, part of the program is mmap'd so copying over the file would corrupt the program's memory (whereas deleting the file would not, due to the way unix handles open files with a 0 link count).