Miners tend to keep whatever coins they think will increase in value, and sell when they see the coin going sideways or down. Botnet owners have little of no overhead, so they just dump like mad. There is absolutely no positive aspect to having botnets on your coin.
1. I would like to see actual evidence of this as opposed to people just stating it. Is there any?
2. Not having a high cost structure would reduce, not increase the incentive to sell. The people who are most motivated to sell quickly are those with high operating costs (electricity, cloud mining, etc.). So this argument about botnet behavior is illogical in addition to unproven.
Re #2
Speaking for myself only, and note my name - when I get free coins on poloniex (via giveaway, trivia contest, etc), I sell them immediately if they have any aggregate value above, say, .005 BTC. The exception would be if I had overwhelming evidence that the coin was rising in value.
That's quite different from running a botnet. A botnet might not have a direct operating cost but you still have to put time and some money into it. Whatever you get out of a botnet, you worked for it.
I general I agree with rpietilla though. There is likely to be little difference in selling behavior as a function of operating cost. Small differences are possible but not that important.