The problem is they're not designing the mag. They're just taking an existing magazine design that is popular, AI mags, and going from there. So they're building the gun around the mag, rather than the mag around the gun, and "standard" magazines like what the Gen1 American comes with are much too wide to fit in the same magwell as an AI mag.
This would lead to some decisions needing to be made such as: Do we support BOTH, selling rifles with both "regular" mags and AI mags? Do we offer the flush mag and an aftermarket DBM kit? What about the stocks, we'd have to manufacture two different stocks if we do two different mag setups? ....
Clearly they decided that the number of people like you and I who don't want the AI mags is going to be outweighed by the people who DO want them.
That's helpful. First I've heard of an AI (Accuracy International) magazine.