I thought they made the announcement 10 days before the fork, not a month.

I'm not sure why you are saying this as if you're defending Coinbase. Warning someone you're going to seize their assets a month in advance doesn't make it any better for them when their assets get seized.

I'm not saying Coinbase is somehow ethically wrong for doing this, I'm saying that one benefit of decentralization is that no one can seize your assets.