They can if you use something like Viglink. That's another reason to roll your own solution. Really don't understand why they decided to go through them just to avoid a bit of regex.
If it's a good fit they can always roll their own later. Hell, they may be rolling their own right now but wanted to start capturing the additional revenue at a lower margin immediately.
>just to avoid a bit of regex
I've never heard of Viglink and am too lazy to google, but I'd wager the real value provided by the company isn't adding affiliate codes, but rather strong analytics features allowing you to optimize revenue across different affiliates etc.
If there's enough money in it then maybe it would be justifiable to put engineering effort into it themselves. In the mean time "free money" for minimal effort.
because it's more fun
Seriously? You think that's a good criteria to judge it on?