This is just selection bias: you won't notice the hundreds of guys who play a bunch against bots and achieve nothing spectacular, but you will notice the one guy who runs extremely hot.
Indeed.
I play almost exclusively against bots on Goko, for the simple reason that I mostly play in situations where I cannot play a full game in a row but can guarantee that I (usually) manage to do my next turn before the time out. In other words, I could not play against humans but bots don't mind my slow-playing.
After quite a bit of games, my isotropish rating is at 28 and has been fluctuating between roughly 25 and 30 all the time. When Isotropic was still around, my rating hovered between 27 and 31. The final rating was 29, after roughly as many games that I have now played on Goko.
Even though I'm just another isolated example, for me the rating system seems to work very accurately despite playing only against the bots, giving me a rating that is practically indistinguishable from what I had on Isotropic playing only against humans (and in general the two ratings seem to be quite comparable). I guess I could improve my rating on Goko a bit by trying to game the bots, but I have zero interested in doing that.
Edit: ...and I would hate to have games against bots removed from the ranking. If I have an opportunity to play against humans it's way better that my rating is roughly at the right level so that I can get right opponents via automatch.