Why must I upload my API keys?
Your Kraken API keys are necessary for the server to place orders on your behalf. The keys are encrypted at the application level, then put into the database. They are only ever decrypted when calling the Kraken API.
If you don't trust the website, good for you. You should under no circumstances share your API keys with someone you don't trust. I'm offering this service for free, so I'm not looking to convince anyone either way.
That being said, I can not personally gain anything from your API keys as they only allow me to place and query orders on your behalf, assuming you use the minimal configuration as specified in a previous FAQ item. I can't, for instance, withdraw your funds to my own addresses.