My apologies. I made an error in my analysis. These rules are not the same. "On <interface>" is not the same as "from/to <interface>". Your rule for $wwwserver uses "to $ext_if", which maps to the address(es) assigned to the external interface itself. That rule does not redirect all traffic.
HAVP is an HTTP proxy.
It would need to be configured as a transparent proxy in order to accept redirected traffic.