I can understand your frustration, I really do. The differences are subtle at first glance, especially to the uninitiated. But it is the customer's responsibility to understand the platform they are using - a hosted blog on wordpress.com vs a self-hosted WordPress site from wordpress.org. The information is out there, but every WordPress user needs to invest the time to understand how WordPress works ( http://www.wp101.com/ is a great place to start). Unfortunately, even with a platform as fantastic as WordPress, creating a site isn't effortless.
Here's a great explanation as to the differences between the two: http://en.support.wordpress.com/com-vs-org/ , and here's a video: http://www.wp101.com/videos/wordpress-com-or-wordpress-org/
This is just a case of an incorrect assumption being made as to what wordpress.com is capable of. In fact, it's not that the themes here aren't compatible with the software on wordpress.com (which simply runs the software available from wordpress.org) - it's that wordpress.com won't let you upload those custom themes as a basic user. It makes sense from a security standpoint - they give you free server space and bandwidth; but they need to control the code that gets executed on their servers. Otherwise you might upload something malicious.
Note that none of the themes in the wordpress.org themes directory are compatible with wordpress.com either - but none of them state "not compatible with wordpress.com". It's just not possible to list everything that a product isn't compatible with. Users running a WordPress site need to understand how the platform works.
Consider this (imperfect) analogy: when a consumer purchases a movie on Blu Ray at the store, there's an assumption that they have a Blu Ray player to play it on. It is the consumer's responsibility to understand that there is some additional equipment required, and that just because they have a television that can display movies doesn't mean they can play the disc.
Here's maybe a more apt example: a customer might buy a new radio for their Toyota. Well, that's great if they own their own car (self-hosted wordpress.org) - they go ahead and install it. If they're renting a Corolla from Hertz (wordpress.com), Hertz isn't not going to let them install their custom equipment. The WordPress platform is the car; wordpress.com is the rental place and wordpress.org is where they can buy their own. Just because the radio is compatible with a Toyota, doesn't mean they can install it in the rental car - and it's not the radio manufacturer's responsibility to explain that ("not compatible with rental cars"). If they want to fully customize, they've got to buy their own.
So, while this is a completely understandable mistake to make, neither the theme authors or ThemeForest are to blame. Anyone undertaking the challenge of building a website takes on the responsibility to invest the time to understand the tools they are working with. Purchasing a product like a WordPress theme assumes a basic level of knowledge about the platform. I know, there's a lot to learn! That's why there are a myriad of resources out there, and there's always the option of hiring a consultant or a freelancer.
And of course, there's always the option of migrating the wordpress.com site to wordpress.org in order to make use of the awesome themes here
Hope that helps!
EDIT: ^ tl;dr