As BirdDog and Alan have noted, the possibility of unlimited internet isn't going to happen anytime in the foreseeable future. The possibility of a higher cap for you, though, may be a different story. They are launching another satellite this year to cover a larger area with Gen4 service. It may or may not alter the possibility of some subscribers being able to get a larger data cap. The largest I can get is the 20/50 plan, as well.
The reason they can't offer unlimited data is due to satellite internet having a much more restricted throughput than land based systems. If they were to have unlimited plans, what would happen is everyone would be streaming all the time and the system would slow to a crawl. So, even though on the surface it seems contradictory to say this, having a data cap actually protects you by making sure that your particular service is usable for you. It can be annoying sometimes, for sure, as we would all like to stream HD movies and such like so many others can, but it's just too hard on the system for everybody to be using so much data.
There may eventually be a day where the infrastructure will allow this, but not for the foreseeable future.
I hope this helps to give you some answers as to why.
What the tech told you about Hughesnet was probably due to incorrect information. I doubt that he knowingly lied to you, though that is always a possibility.
With regard to the other, it may very well be what BirdDog talks about. There's a plan underfoot to have a slew of low earth orbit satellites that would link together and provide internet to places that wouldn't traditionally be able to get it. Even still, I don't know that it will be unlimited, because as time goes on and apps and things require much more bandwidth, even a much higher capacity system may end up beginning to suffer from the same problems Hughesnet would if it was unlimited, especially with a large number of people using it. Even as we speak there are landline offerings that are beginning to add data caps, although considerably larger than what Hughesnet can offer. It's like they make something that is supposed to be able to supply unlimited data to all of their customers for 20 years, then as time goes on they realize that their calculations were ten or more years off.