I could have sworn I'd read it was down to legislation but now I look I can't find anything.It's not EU/UK law that restricts noseweights, but it seems common in EU/UK for noseweight to be limited to 4% of the towing limit, which is a EU/UK minimum. The Aussies and Americans run heavier caravans AND expect higher noseweight (tongueweight) %, typically 10%. Early versions of Touareg had a 350 kg limit out there but reduced to 280 kg on the T4 - compared to 140 kg limit on Touaregs sold in EU/UK.
Most caravans sold in EU/UK have a 100kg limit applied to their hitch anyway.
Wonder if there's a design difference between the Auz/US tow hitches and spring ratings and EU's. Or it's simply a stated rating change, as (as you say) our vans have lower max nose weights?