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 
No fees are being paid (check the links to blockchain,info).