How can you prove and guarantee that all bitcoins get forwarded instead of kept secretly in your wallet?
If your website is somehow hacked, the hackers would have tons of unencrypted private keys.
Also, "Case 2" - you should not provide any illegal services
If our website is hacked, all they can receive is the IP address of our one of our off-site nodes (which doesn't help them whatsoever), and a list of hashed IP addresses who have used the website in the past hour. There is no potential for a major security breach simply by hacking our website.
You can guarantee that our website is trustworthy by testing us out with a dollar. And then work up to two. And if everyone does this, you'll quickly see that we actually forward. If you don't trust us, use us with smaller amounts until we've forwarded hundreds of bitcoins (adding a counter on the website soon).
We couldn't secretly keep them in our wallet because transactions are public (and can be viewed on the blockchain). You can verify that we are forwarding all your transactions to your address. If we ever stopped, all you'd have to do would be to stop sending bitcoins to your forwarding address (although this will never happen).