Because the standard requires it:
3. If file is not of type directory, the
-foption is not specified, and either the permissions of file do not permit writing and the standard input is a terminal or the-ioption is specified,rmshall write a prompt to the standard error and read a line from the standard input. If the response is not affirmative, rm shall do nothing more with the current file and go on to any remaining files.
So a) this is a matter specific to the rm utility (it doesn't say anything about how permissions work in general) and b) you can override it with either rm -f file or true | rm file