Is it fair to take down a friendly level 10 gym to 49500 (using a different team) and then repower up the 500 pts to slot your own Pokemon at the top? Seems thats what everyone is doing now.

Honestly... what do you think? If you have to ask...

On the other hand, there are a lot of things about the game (and life in general) that are not "fair" but that rarely stops anyone from taking advantage when they can. It's also crummy when an opposing team lets someone else to do the dirty work of taking down a gym, waits for spots to open up, and then snakes them after having done nothing to earn them, but it happens all the time. There is still enough anonymity in the game such that you can hide in the shadows and do stuff like that without having to face the person you're screwing over.

I still believe in the old golden rule, "Treat others the way you would like to be treated," but I gather that I'm in the minority.

In the term of usage, it says one can not has multiple accounts

Is it fair that I've been sniped on 4 of my last 6 level ups on a friendly gym?

Is it fair that I lost both an Exeggutor and a Blastoise within the last hour when they both spawned on the same exact spot that my GPS absolutely refused to move towards even when walking around block 3 times? (in NYC, GPS drift is horrible)

No, it's not fair. But sometimes that's just how things is

If you're doing this to replace the lowest CP pokemon with a much higher CP to make the gym a lot stronger, it is absolutely fair, and nobody should be complaining about that. Especially when the previously lowest CP was a good 500+ lower than everything else.

But if you are having the lowest CP Pokemon taken out only to put your own similarly strong defender back near the bottom, that's just being selfish and not helping anyone but yourself. And you'll probably just end up getting knocked out in a similar way later, so it's pretty pointless.