It is important to realize that a rule is really a command transformation mechanism, or command macro. The transformation happens before the execution of the command starts. If you actually want an operation that fires independently for each physical row, you probably want to use a trigger, not a rule
Thus, I should not have to use a trigger for TRUNCATE because the "each row" concept does not apply. Plus it makes perfect sense to want to transform the truncate command and transform into ignore
Just in case it has not been made obvious yet, rules are silently deprecated. They still exist because views depend on them, but it is generally considered best practices to not use them outside that realm. So if you want the rule behavior to change for TRUNCATE(if that is even possible) you are fighting an uphill battle. You may pursue that fight of course, but I would think you will get a quicker return on your time if you just forget about using a RULE and stick to a TRIGGER instead.