First off, the reporter himself says the footage was poor footage anyhow. He didn't record anything worthwhile.

Secondly, the evidence comment the reporter is so puzzled about isn't all that puzzling. The cop isn't saying he wants evidence destroyed. He's saying that that footage may contain evidence they are gathering and that they don't want it entering the public realm until the case is processed and what not.

That's pretty common practice. Now whether he had a legal right to actually make the guy delete the footage, well, it appears he did not have that right. He overstepped his boundaries there.

Which is another thing cops tend to do on an all too regular basis.