Not all front-running is illegal, see this article which discusses a few examples, including quote matching and order anticipation: https://blogs.cfainstitute.org/investor/2013/04/24/what-to-d...
Front-running is very common in bond trading.
Front-running is very common in bond trading.
They are market-making, which is something else entirely than front-running.
It's often claimed that HFT help make markets more liquid and efficient, "which helps everyone".
It's much less than clear whether the presence of HFT helps make an entire market more efficient, or whether just the HFTs are able to benefit.