There's two ways to make money in the game. One is to have a high population on a world. This is very difficult, but possible, and it requires the most advanced morale buildings (and possible several morale resources), as well as a large planet with lots of tiles. If you can get past 25-27B (or whatever it is that is the cap), and still have a decent tax rate, then you can make money. Note that this is not easy or often viable, particularly if you wish to retain a 100% morale (for the population growth) on said planet. But if you can pull it off, then that planet can be your money maker.
The other is to have relatively little population, but use taxes and economy buildings to make your money on these worlds. You can pull this off with a much smaller planet, but you won't be able to make the money you could with the above method. The problem here is that you could, with a linear scale and some good morale improvements, literally hit 100% taxes and still have decent morale. The 80% discontinuity exists to keep your profits, while very large, still within some form of reason.
Yes, both of these are what a programmer would call "hacks". They're patches to make a specifically designed system work when someone finds a way to break it. That's one of the downsides with a more free-form building system like GalCiv2 (as opposed to the way Civ games work, where you can only build one of any particular building in a city): if you let poeple compound bonuses, they may find ways to achieve things you never intended.
If you didn't have the first hack (ie, the eventual stopping of morale drops due to population), then the first method of money making would never work nearly as well as method #2. It already requires a big planet to make it work as it is, as well as the most expensive morale building. Even then, it requires a huge population (35+

to really make it worth the effort. The other method can pay off with 17B people.
If you didn't have the second hack, you make money method #2 preferential, because you can hit 100% taxes and make that much more money. Whether this is "reasonable" or not is irrelevant; this is a game. Balancing the game means that both methods of revenue generation should be viable. 80% was considered a reasonable cap on taxes.
I would like to point out, however, in one of my games, I could hit 81% with only a drop to 67% morale.