Generally, illegal trading resolves around the idea of trading on non-public information. And front running falls under that category (access to order data that other participants do not).
However, HFT's do not trade on non-public information. Every participant has access to the same market data. I could start my own "HFT firm" tomorrow; I would just be incredibly unsuccessful at it because I don't have the finances or computing resources to execute.
Front running is illegal. However, making a better price prediction than the rest of the market and trading on it is not the same thing as front running.