Probably because they keep cans of air in the back room.

The new one? I was told they would have to send my 1 month old, purchased from an Apple Store late 2016 MBP in for a flakey 'R' key to have the top case replaced. I asked if they could pop off the keycap and clean it and they said that they do not do that on the butterfly design.

No, mine is the late 2013 Macbook Pro Retina. The parent poster said "Apple doesn't repair anyone's laptop on site".

I read "anyone" in this context to mean Apple is not discriminating against people who purchased from 3rd parties.

This is in response to the GP's comment "They had the gall to tell me that since I didn't buy it directly from Apple, they wouldn't (not couldn't, wouldn't) repair it in store" - which isn't the case, this is the policy for the butterfly models.

Replacing the key and butterfly mechanism doesn’t always work :(

I swapped my keys around for Dvorak. I moved almost every key, and they all still work. But, I can see their hesitation. It’s really easy to mess up.

I think what he meant it is doesn't always fix the problem for problematic keys.

For me, it has always fixed problems of keys that stopped working, but it doesn't seem to necessarily fix keys that double press. I assume that is related to an issue of the membrane itself or something under it.

Depends on the country / store tbh. Canadian Apple Stores don't send any Macs out to repair centres. US stores generally do, but I think they still have the latitude to do some repairs in store.