Because it wouldn't do the trick just the same. If they are sending transactions just a few blocks apart, then 5 BTC wouldn't be able to be sent without a fee. Remember that a fee is required whenever Bitcoin * Days is less than 1. 30 minutes * 300 BTC = 6.25 Bitcoin Days, but 30 minutes * 5 BTC = 0.104 Bitcoin Days.
So would 50 BTC be able to be used to do the same thing then?
Its not spamming if they are paying fees ![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
No fees are being paid (check the links to blockchain,info).