My guess is that since the packet is officially reserved and should not be set, a common firewall or other security appliance considers said packets to be malformed and drops them as a default behavior.
The evil bit could be something of a self-fulfilling prophecy. Because no one uses it, that makes it a source of bugs/vulnerabilities; therefore, anyone setting it deliberately but not maliciously (such as for a joke) will want to turn it off; only those who want to exploit it maliciously will keep it turned on; hence, anything with an evil bit can be safely assumed to be, in fact, evil, and it should be filtered out automatically.
> Devices such as firewalls MUST drop all inbound packets that have the evil bit set.
> Packets with the evil bit off MUST NOT be dropped.
That seems to be at odds with standard firewall operation, that may choose to drop packets because of all sorts of reasons unrelated to the "Evil bit". This would seem to constrain their operations unnecessarily, and so I would say that it is in the best interest of security vendors to ignore most of this RFC as frivolous and not binding.
Otherwise, we're back to square 1, where you don't know what caused your packets to be dropped even though they are clearly and explicitly not evil!