They... They still attacked first.
That's the part that matters.
Paging Han Solo to this thread for a discussion on why shooting first is a smart idea.
Han Solo was threatened first. He didn't just start firing as soon as Greedo showed up. Plus, he knew who Greedo was and knew what he was likely to do. The same can't be said for those mining guild TIE pilots. They were sent after the pergil, saw a freighter, and started firing on them without even hailing them. Seems kind of stupid, really, because that could have been a potential customer for them...
as for the Mining Guild - most private military contractors in the employ of a tyrannical empire will operate on a "shoot first" policy because they will be targets for exactly what happened. Go see if a Blackwater facility in Iraq will let you wander in and buy oil or guns