OK, I purchased a 2013 WKA tech manual with a parts order so I could get an official explanation of what is and isn't in the rules
For 4-cycle karts (Gold Cup rules), the diagram Jim included is the rule. Two cycle karts (Manufacturer's Cup) rules DO NOT require the hubs to be secured to the axle.
So my read on this is that everyone running Animal or WF engines needs to comply, and Yamaha and Tag guys do not.
Our rules do not require this. Unless there's an insurance issue (possible), shouldn't we be teched to our rules?
PS: Huge fan of us having our entire rulebook and technical regulations freely available. This whole "we run to WKA regs except" thing at Avon basically means you need to buy a book in order to ensure you're legal. Not great.