Well it’s been a while since I studied law, so don’t take this as gospel, but…
The terms and conditions of the site, or pretty much any site, form a contract between Envato and the buyer, Envato and the seller, and/or the seller and the buyer. In the majority of countries, most likely the US and Australia included, no contract is allowed to supercede the law of the land. In the majority of cases, there’s very rarely anything specific in a contract that does supercede the law in its own right. It’s the circumstances of the case that might result in the law taking precedence over the contract.
So in your example, copyright laws protect the work of designers, musicians, programmers etc. If there is a breach of copyright, then the person who’s rights have been infringed has the right to compensation as set out by law and by the courts. If the legal owner took the buyer to court, then as the buyer had no reason whatsoever to believe they were licensing a stolen track, they would most likely be in the clear. Then the legal owner would probably sue Envato and/or the seller.
Seeing it’s against copyright law to sell work that isn’t your own, then the $4 limitation wouldn’t apply, as the contract is not more important than the law. With the “I certify this work is entirely my own…” checkbox when submitting items then most, if not all, of the blame should lie with the seller, and they’d be liable for the full force of the court’s wrath.
At the risk of stereotyping an entire nation, people from the US are probably more likely to instigate litigation than most other nations, so this might be a proactive step to minimise any potential issues before they occur.
As I say, I’m not a lawyer and I’m only making assumption here, but I’m guessing this is more of a crossing the t’s and dotting the i’s exercise, to be on the safe side prior to the move to the US; rather than some attempt to shirk responsibility when there is an issue that truly warrants redress. It’s the stock marketplace equivalent of adding ‘right of admission reserved’ to a flyer, or ‘errors and omissions exempt’ to a sale poster or ‘please check your change before leaving the shop as mistakes cannot be rectified afterwards’ etc etc.
That’s the way I see it anyway.