The BTC paid during the Ethereum crowdsale went to the Ethereum Foundation bitcoin address, and that address can be audited. Misappropriating those funds would leave a massive chain trail, and require a massive conspiracy to plant the Ethereum Foundation with co-conspirators. It would be unthinkable.
On top of all of that, the operators would need to wait years to retrieve the BTC to not draw attention to the scam, meaning the assets would be locked up and out of reach. So even under this fantastical scenario, the ETH wouldn't be free: there would be a very significant liquidity cost in funds lockup, not to mention all the planning and risks involved in participating in a complex crime, involving an auditable foundation operating under Swiss law.
Where are the transparency reports detailing how the 31529 BTC raised in the crowdsale were spent?
It would seem straightforward to spend a large fraction of that on software development which would mean that the creators can take part in the crowdsale (which is largely anonymous) and have the spent BTC flow back to themselves.
They would know that audits could always be demanded, and that an on-chain trail would exist. If said audits reveal massive grants being given to themselves, they would immediately fall under intense scrutiny, and it would only take one of their co-conspirators to spill the beans for them to all be burned.
For them to actually consider this subterfuge, with the risks involved, would be unthinkable.
While Ethereum started out as PoW, its huge premine still constitutes a big trust issue.