Spam.
Transaction costs are a method to minimize this.
Doesn't seem to be working at present.
It is because the fee is way too low for such spamming transaction.
Take 0dd3c35d3fe71effcf638bc49d15e076c6262e356eeba4368f9ea6eef2dd74b8 as an example.
The spammer sent 1 satoshi to 70 different addresses and paid 0.0004 btc as transaction fee.
So, the ad to 70 different addresses just cost him 0.0004007 btc (or equivalently $0.16).