do not hide an event if one of its categories is deactivated, ...

...only if all of them are deactivated. I.e. if at least one category of an event is activated, show the event.

See https://github.com/syslabcom/staralliance.webwork/wiki/Business-rules-events

Also @cornae voiced the opinion that in general it is more natural to apply an _OR_ to css classes than an _AND_.