There were 2 reasons people who purchased it early got it for free, if I recall correctly.
At the time, people who were buying MJ were promised a free upgrade to the next version. (when I bought 7 I was promised 8, for instance) But the decision to split the product into two product lines hadn't been made yet. When it had, and the price difference was decided upon, something had to be done for those people who already purchased MJ8 under the promise of a free upgrade to 9. They were grandfathered in, if you will. I believe the current upgrade scenario is that users who buy any major version, only get the minor upgrades for free now, and the next major version will require another purchase. This was the biggest reason.
The second reason, IIRC, was because people who had purchased MJ 7 and were beta testing MJ 9 wanted to buy MJ 9 (again this was before the decision to split the products into MJ & MC) and the code to do so hadn't been written & finalized yet. So JRiver allowed people to buy MJ 8 and that license would count as a purchase of MJ 9.
Once the decision to split the product up into MJ 8 and MC 9 was made, some people, who had already purchased 9, decided they'd rather stick with 8, and so JRiver made it possible for licenses purchased in a certain time period to be used on either product.
Basically, what you've run into is JRiver's attempt at doing right by as many people as possible during a time when product lines and licensing terms were changing. Personally, I think they did as good a job as they could. Because if they then said "You're right, since people who purchased 3 months before you got to upgrade for free, then you'll be able to as well." they'd have to effectively give EVERYBODY a free upgrade and they'd never be able to change their licensing terms.
MC 9 was advertized on the download page for MJ 8. You yourself admit to seeing this on the download page. It's unfortunate that you chose not to click the link and read up on it. If you had, you'd have seen the deadlines for buying 8 and getting 9 for free were spelled out very clearly. They gave people a lot of notice that the change was coming and a lot of time to decide to buy 8 and get 9 for free if they wanted it.
I realize that none of this will probably make you feel any better about the situation, but I hope it will at least help you to understand why and how things got to be this way. Transitions are always tough, both for the company and the customers, and JRiver struggled for quite some time with this decision. They even brought it up for discussion in the forums - both the decision to split the product and the new licensing terms. It's clear they are doing their best to accomodate as many people as possible, but as I said before, they have to draw the line at some point. Unfortunately, you found yourself on the wrong side of that line.