I imagine the rationale was something along the lines of many servers/apps are written to incorrectly return 200 with a descriptive error page rather than 500 or whatever the appropriate status code would be. And at the time ab was first written, pages were a lot more static than they are now, so a different page would be more likely to indicate an incorrect response.
[1]: http://stackoverflow.com/a/579466/94830
[2]: https://issues.apache.org/bugzilla/show_bug.cgi?id=42040
I imagine the rationale was something along the lines of many servers/apps are written to incorrectly return 200 with a descriptive error page rather than 500 or whatever the appropriate status code would be. And at the time ab was first written, pages were a lot more static than they are now, so a different page would be more likely to indicate an incorrect response.