I'm trying not to see the FWS as an enemy. I DO think they're misguided, and acknowledge that there may be some overlap of individuals between FWS and some more radical animal welfare groups, but FWS is ultimately in place to "...conserve, protect, and enhance fish, wildlife, and plants and their habitats for the continuing benefit of the American people."
The rule was heavy-handed. It's easy to look at a practice and say, "well let's just be done with the whole thing" rather than find the resources to correct and manage each aspect of it that would ensure the concerns do not actually become issues.
With the way things stand, that seemingly arbitrary and counterproductive qualifier makes the distribution of reticulated pythons regulated and enforceable. There's less of a draw to do things illegally - why pay $500 of fines (or however much the penalty is) when you can pay $250 to join USARK? And those that ship legally will be policed by USARK itself, just like we try to do with CITES permits. No one wants to be associated with someone who is irresponsible or a crook, or risk losing what they've worked so hard for.
Essentially, by "allowing" USARK to ship large constructors, FWS is putting the accountability on USARK and if a bunch of people skirt that rule, it'll look bad on the industry as a whole (and risk losing support for future battles) since there's a solution that accommodates any claims of desperation. Any non compliance looks that much worse since we have something to prove.
The FWS and USARK ultimately want the same thing: for giant constrictors to not destroy native wildlife or cause mass panic because one of them ate Mrs. Doe's teacup yorkie. This is just one way of forcing a compromise, rather than saying "okay, well then you all need to have permits and take these classes and comply with these inspections..." which eats up a bunch of resources FWS would probably rather use elsewhere.