Almost every commercial satellite is just a repeater. They have a sensitive receiver and a powerful transmitter, a couple filters, and basically is that. They almost never employ any encryption at all, because sending a computer in the space is expensive, as more weight means more money. And you cannot just send your average Intel processor over, you must design a radiation resistant computer, shield its components with heavy metals (adding weight). So no, just send a repeater.
The security of the satellite is based on the difficulty of interfering with it. Anyone trying to jam a satellite will need a powerful transmitter, a good antenna, and internal knowledge of the receiving band of the satellite. Knowing the frequency is not enough, they need to know the protocol, and that is not just a Google search away. Most of the satellite communication hardware is protected by patents and industrial secrets. They are not usually available.
A radio antenna is not like a laser bean. It radiates on every direction, but with different power levels. It's something called Radiation Pattern. Any good antenna will radiate almost all power in the desired direction, but some power will "leak" around the antenna, even behind it.
Another difficulty is the power level. The power needed depends heavily on the bandwidth. You can send a few bytes per second over a 25mW transmitter to 700km away, but you cannot transmit HDTV to over 35k km on a sub-kW transmitter. And that is expensive and difficult.
So, a transmitter powerful enough to send data over the upper atmosphere to a satellite receiver will leak a lot of power around his antenna, not only on the main frequency, but on the neighbor bands too. That will interfere with the communications of a lot of people - the ham radio operators. And they are professionals, they hate people interfering with their spectrum, and they have knowledge, equipment and motivation to detect the source of the disturbance. Try buying any cheap transmitter on eBay, modify its amplifier to output more power, and let it transmit something for 24h. Someone will knock on your door. Fuming. So don't do that.
So while it's possible, no attackers will do this because:
- it's very expensive
- technically difficult
- getting caught is very easy
- the fines for doing so are astronomically high
- and the most important: what would they gain?