This was the result of a no-knock warrant: http://en.wikipedia.org/wiki/No-knock_warrant , which allows police to enter a home without prior warning, on the theory that giving notice will allow people inside to destroy evidence. They are almost uniquely a product of the drug war. One can imagine very few situations outside of drugs were evidence could be so easily destroyed as to justify a no-knock warrant.
The risk of some evidence being destroyed versus potentially killing a kid (and in this case 'luckily' only wounding that kid) seems to be an easy choice. To protect and serve and all that.
Why isn't this headline news across America?