This isn't about a mint stealing funds from its own users. It's about a malicious mint stealing funds from users of another mint. E.g. you trust Alice's mint and have $1000 on there. You don't trust Bob's mint, but someone airdrops you $5 in ecash from Bob's mint. You try to transfer (melt) the ecash to Alice's mint (which you trust) but doing so allows Bob to steal the $1000 of valid ecash that you already had on Alice's mint.