If you post your BTC tip publicly, someone could enter the site and fill out the form, "on your behalf", and exchange it to ETH instead, since there's no other (practical) way to validate whose tip it is. A possible "attacker" would gain nothing from this but just annoy people.
Ah, I see. Is there no plan to add a verification or something similar to prevent it? I guess the user can simply never post it publicly, but it would be interesting if there was an additional check before somebody made the transaction even in private. I guess that's doable with account registration or something similar, although it would reduce the user experience. I can see people abusing this to discourage using your service, at least publicly since they'd need to do multiple transactions to convert it to the right coins if they fall to such an attack.