Why are they using a large amount of coins for this purpose? Couldn't they just transfer 1 coin back and forth and acheive the same result?
A good question - does it have something to do with the minimum tx fee algo or maybe the spammer is just "showing off" how much BTC they have?
The free TX policy has a coin days destroyed. For example, sending 100 BTC around would allow you to get free TX 100x faster than sending 1 BTC around.