The TV crews were there of their own volition. Somebody tipped them off and they showed up for the raid. The ATF did not invite them.
The ATF is a federal agency and the execute their own search warrants - just like the FBI. The don't turn it over to the local sheriff - it's a jurisdictional matter. They doesn't mean they can't ask him for advice or to tag along, but every police agency executes its own warrants, since they are responsible for what happens.
Also, the ATF did not execute the warrant with armored cars. They approached the compound in cattle trailers towed by pickup trucks owned by individual agents.
The armored cars were not brought in until towards the end of the seige - long after the shootout occurred.
The ATFs big mistake was that they lost the element of surprise and conducted the raid nonetheless. That happened when the press asked for directions to the compound - from David Koresh's brother in-law, who promptly informed the Branch Davidians. That gave them time to arm up.
The ATF was entirely correct in their assessment that the Davidians were violent and would resist. Body armor and heavy weapons were entirely justified. But once they knew the Davidians knew they were coming, they shold have postponed.
http://en.wikipedia.org/wiki/Waco_Siege