Ask HN: Why are websites warning me about their cookies?

It's a (slightly excessive) reaction to the EU cookie law.

IANAL but as far as I can tell, it's only required when you have cookies from third parties outside your control like Google Analytics.

If you only use Cookies to login it should not be necessary.

European Union regulation: https://www.google.com/search?q=eu+cookie+law

It's a law in some countries.

Where it isn't--remember that some people don't "trust" them.

EU law & regulations require that you warn about cookie usage.