Hacking is actually a broad topic. Despite what stock photos of people hunched over in hoodies may have you believe, there is actually a very large ethical hacking community. Some ethical hackers probe for holes in software in an attempt to find exploits and inform a developer about them before one of the bad guys does. Some of them do it for fun, either in private, in small groups, or at gatherings known as hackathons. Others make a living that way. Tech companies like Google often offer very large bounties to hackers who can crack its software. Then you have the illegal stuff, which is also a big money maker but one that may land you in federal prison for the rest of your days if you get caught.
As for hacking satellites specifically — that can also be legal if you tick the correct boxes first. Even after they’re decommissioned, satellites are still someone’s property. If the owner discovers what you’ve done, you could land in legal trouble. Then there’s the whole list of regulations that come with satellite operation and use. So even if the satellite’s owner doesn’t care, you can still find yourself falling foul of the FCC.
The hack performed by Koscher was entirely legal. Before it was performed, a lease was acquired for the satellite’s transponder, which is the part that manages what information the satellite is sending and receiving. Koscher also acquired a license so he could use the abandoned uplink station selected for the task without falling afoul of authorities. If you too want to legally hijack a satellite, there will be a similar amount of paperwork and a few fees involved. But once that’s out of the way, things may actually get less complex.