Which particular instance of HFT firms getting early data are you referring to? Different people might be talking about wildly different things when they say that.

The general answer is going to be: the HFT's early access doesn't implicate a principal/agent problem. The people generating the data HFTs act on don't have a fiduciary duty to the shareholders of the companies they report on.

Ah okay this makes sense. I didn't think about the problem as a principal/agent problem, which is how insider trading is framed. Got it. Thank you.

HFT firms don't get data early[1], they just can act on it faster than any human can.

1. There have been a few cases where data was available seconds early on certain feeds, but those were mistakes.

Interesting so as long as the data is released to everyone at the same time, it doesn't matter if some people are able to acquire that data before it reaches others. As tptacek pointed out below I didn't think about it in the principal/agent framework.