Some people seem to think that cats need to be able to drink a nice saucer of milk. They don’t realize that this can lead to a messy litter box.
In small enough amounts milk fine.
They don’t lose all of their lactose production capacity, it just drops significantly. That said, cats don’t drink milk for the sugar, they do it for the fat, which is why they’d prefer and it’s probably better for them, again in moderation.
If you must give them milk, make sure that it is blended with cream. It is probably a bit better than straight milk, but cream is still best.
Milk is one of those cliches that got started back before people cared to treat their animals right.
Lactose isn’t the only component of milk that causes gastrointestinal distress and cats. Some studies have looked at hydrolyzed (broken apart) protein lactose free milk and even with those formulations cats were not free of clinical signs. Safest bet is to avoid all but small occasional servings of dairy products.
There are plenty of things that just seem to grab on and refuse to let go, like carrots and night vision or lemmings jumping off cliffs, past the point of where the original story can be told bringing any kind of sense to the scenario.
Unfortunately for this one it is unlikely to die anytime soon.