Suppose someone is interested in contributing more than 50 ETH? How do you enforce that since people are just sending funds from their wallet to yours? They could indicate that they want to send 50 ETH, but in reality only send 5 ETH. You would still have to honor sending them tokens as well. Isn't this just a "pre-sale"?
We are limiting the amount to 50 ETH to be in-line with why we are doing a white-list: to allow as many of our community members to be able to buy AVT as possible. If we allowed larger amounts, we would run the risk of the whitelist being oversubscribed with "whales". The contracts are set up to only accept the exact amount of Ether from an address as specified in the whitelist.