The questions you asked have some obvious answers.
Sure they can upgrade the hardware.
Nothing technically stops them from offering unlimted data.
Why charge for how much you download? To make money.
Since they have many people looking for service in an area with no other options, they can do what they feel they need to to make a profit. They also need to pay for the bandwidth they use to some-one, if they offered unlimited service to everyone they'd end up with huge slowness issues, satellite is a really inefficient way to provice net access, but if it's the only way in your area, not much that can be done.