Maybe they are just friends.

Man, is there anything the Japanese can't do?!

Don't you mean "OMG dead mouse"?

That's one confused cat.

So they make the mouse stop smelling so it can't smell the cat and know what it is? How does the mouse smell to find food and friends and stuff? And they think they can do this to other mammals so they don't fear their natural enemies? Why?

The mousie still has its sense of smell, it's just that the smell doesn't set off the cat-alarm bells in its teeny brain that it ordinarily does.

I'm not sure why they wanted to do this, though, and what possible applications it could have. Aside from making super-cute photo ops...