First of all, this is a great reason why you don't keep coins on any exchange for a long period of time. When the coins are there, it's not your money, instead, it is the exchange's money and they can easily take it away from you like here.

I personally think both parties are at fault here. If you keep amounts of money on an exchange, I'd assume you trust the exchange and would often use it and follow up on your accounts.

There are no legal codes that require exchanges to send you notifications, but I would agree and say it would be a good update. As long as they have given you fair notice via a fairly visible option (their site), it's fair. Think about getting evicted as a good comparison here.