No, I don't think it's mean at all.
It may seem rather odd to some people, but sometimes, some cat owners WILL keep their cat(s) indoors for the sake of their safety. This may be the case especially if the area which they live in is not a particularly nice area, or if it's near a source of danger to the cat(s), for example busy roads with lots of traffic, or if lots of dogs come past and a cat who they own doesn't get on with dogs. Also, believe it or not, some cats actually prefer to spend time indoors out of their own personal choice. If they have a close bond with their owner, or one owner in particular for instance, they may be inside a lot to spend time inside if it's with the person they're fond of.
Anyway, it's totally fine for the cats to just look outside through a window if that's what they want to do, because it keeps them entertained but safe at the same time. It's like a little child watching cartoons online or on a TV in order to keep them entertained, so they don't get bored.