I don’t have first hand info on all of this, but I believe it was something like… I’ll sell you this car with this battery(non traxxas) or you need this size battery without the traxxas connector so I can do this for you…..
I know here locally Traxxas has revoked a dealers status because he wasn’t always selling their batteries with their vehicles. It’s said that he replaced the connectors with other ones as well so they could be universal.