3.5.9. Anvilling is blocking a weapon strike by laying a weapon against a target area and/or shield and is illegal.
3.5.9.1. Sufficient force hits must be taken through anvilling weapons.
3.5.9.1 seems to allow for anvilling so long as you actually take the shot that was anvilled.
If we look at some other instances where we use the word "illegal", it's easy to tell that these behaviors would probably cause safety or playability issues:
3.11.3. Intentional shield contact to the head or neck is illegal.
3.11.4.6. It is illegal to bash combatants that have bows and/or arrows/bolts.
3.11.7.4. Shield kicking from the rear is illegal.
1.1.8. Intentional illegal acts may result in immediate removal from the field, until a marshal allows them re-entry. Marshals are the arbiter of intent.
In my opinion, anvilling does not pose a safety issue or a playability issue so long as 3.5.9.1 is obeyed and you take the shot through the anvil. Anvilling is not a reason to kick someone off the field, so long as they take the hits appropriately.
Suggested edit:
3.5.9. Anvilling is blocking a weapon strike by laying a weapon against a target area and/or shieldand is illegal. 3.5.9.1. Sufficient force hits must be taken through anvilling weapons.