Well it truly is very simple:
The purpose of opening an affiliate program is to bring fresh traffic to the site, that is... people who have never been exposed to the site will see it and a portion of them will eventually become customers. Once someone becomes a costumer the affiliate that brought that person to the site gets a 20% cut of anything he spends for life. This is a high percentage, consider the site has to then manage to pay models 50%, pay the card processors 15% and cover the running costs (bandwidth, staff, platform, etc) with the remaining 80% (only model and processor fee already adds up to 75%) so we can basically say that for every customer that comes to the site through an affiliate link the site breaks even or makes very little profit, but it's still worth it because on top of getting these users they also gain in visibility... these new customers will bring friends through word of mouth, etc.
What happens when models are allowed to use this feature? Well... some models have huge social media accounts with thousands of followers who have never been exposed to camming who mainly know her from their social media. Most models do not. So what happens is models end up cannibalising the website's new fresh customers that did not arrive through an affiliate. Say... someone looks up "Drop dead gorgeous brunette goddess with a phat ass that will make me weak in the knees" on google and somehow find my camroom on their own. If this person signs up then CB gets 100% of his spending. But what if instead of signing up at once, he follows me on twitter and I post my affiliate link tomorrow and he clicks on it, and THEN signs up? The site will only get 80% of his spending, the rest will go to me. But why? I didn't bring this member to the site... he would have signed up anyway... but because I am featured on the site AND I am allowed to have an affiliate link I can make the site lose a big percentage that should not belong to me.
Now.. there is also the other side of the argument. The other side says... well... the above is true BUT... how do you know the member would have signed up anyway? By allowing models to use the affiliate program you are giving models the incentive to push hard for member conversion, which they would otherwise not do. This is a good strategy if you have massive traffic like CB and want a fast member conversion rate. MFC however has little traffic and they depend a lot of word of mouth so when a model steals the affiliate percentage it really hurts their bottom line.