Just a couple points.
Most fighters fight 2x per year because they CHOOSE to fight only 2x per year. The UFC standard contract (confirmed by Alvarez's contract and interviews from agents/managers that have had fighters go to the UFC) is that the UFC is required to offer 3 matches per year. It is up to each fighter to take or not take those matches. The UFC is only required to OFFER the fights, they don't have to actually MAKE 3 fights per year.
If the UFC offers 3 fights and the fighter turns them all down, then tough luck. They held up their end of the bargain.
Many fighters choose to NOT take more than 2 a year because they are waiting for a better fight. Some are forced to not take 2 per year due to injury of course, but that's not the majority. Some choose to take more.
Cerrone fought 5x in 2011, 2x in 2012, 4x in 2013, 4x in 2014, and has already fought 3x in 2015. In other words, the opportunity to fight more is often there.
Additionally, fighting 2x per year means you're dedicating 20 weeks a year between camps and the actual fight. There's nothing stopping those fighters from taking additional employment such as teaching MA, general side jobs, etc.
Now before people start saying that the UFC is the NFL/MLB/NHL of MMA and should be providing similar compensation, the UFC as an organization is the major leagues, but the entire roster is NOT the major leagues.
The UFC essentially has two tiers of fighters, main card (pros) and prelim fighters (semi-pro). Think of main card fighters as the NFL/NHL/MLB in that they're televised to a wide audience and the athletes, have some level of name recognition and are recognized as the "best".
The rest of the fighters are more like the D-league in the NBA ($30k/year), AAA hockey ($25k-$30K/year) or the CFL ($50K/year minimum). And the UFC is several orders of magnitude less in revenue, viewership than any of the other major sports.
And you are right, the UFC does not own the fighters. Thompson walked as did Koscheck. The reason why more fighters still choose to come to the UFC is that unless you're an already established name then you're simply going to get paid more fighting for the UFC than organizations like ONE FC, Bellator or WSOF.
Now all that doesn't mean that the Reebok deal doesn't suck for the fighters. It does suck for the fighters. However it is within the rights of the UFC to impose uniforms during UFC events. While it's not "fair" to the fighters within the context of them having an outside income stream being taken away, it's not unfair in the sense that the platform provided by the UFC does belong to the UFC.
The main thing now is what's done is done.
The fighters aren't going to get that advertising space back no matter how much they complain. The rights were already signed away in the Reebok deal. The fighters need to worry about what to do next, which is either to negotiate for higher base pay individually or collectively, or to see if they can seek better compensation with another organization, or to seek income streams from other sources outside of their fighting (other jobs, other sponsorships, independent businesses, etc).