That's not actually slander/libel. Truth is an absolute defence, and that does not require you to disclose details up front. You'd only need to demonstrate truth to defend yourself if sued.
In this case I also expect it's all very carefully worded ("Be careful! This site might be trying to harm your computer") to be legal even in cases when they accidentally (and inevitably) miscategorize a site.
Whether "truth is an absolute defence" depends on the jurisdicton. In Canada and Britain proof of truth is an absolute defence, but mere truth is no defence at all. Think about it - how much of what you say could you actually prove in a court of law?
In court and parliament, this is relaxed somewhat. But just 'cause you - say - saw a murder by X in broad daylight, doesn't mean you get to say you did anywhere you like, in Canada and Britain.
PS, yes defence is really spelled with a c in Canada. This was deliberately done historically in order to distinguish ourselves from the US long before the internet and spell-checkers.
IANAL - but then a lot of lawyers aren't much good at their game either.
Yeah maybe, but the reality is they won't. You can shake fists at MS all day long but it won't get your business up and running. Fixing the breach (if it's not a false positive of course) will. It's not right or fair, but it is reality, and you can only control your own actions.
In this case I also expect it's all very carefully worded ("Be careful! This site might be trying to harm your computer") to be legal even in cases when they accidentally (and inevitably) miscategorize a site.