Selling elec stock at a supply house way back in the 80s. I had to ask the purchaser if they planned on using it for a sub panel, then sell them an addition ground bar kit. I use to remove the connecting bar on panels myself to isolate, my understanding is this removal exempts the panel from it's original equipment and becomes void. I believe that's why there's threaded holes for placement of a gnd bar. I can understand why this would upset someone that it's not more deversafied, but if the panel manufacturer added a ground bar kit with every panel it would be more expensive, and it wouldn't always be used, so then the constumer would be upset for paying more for something they don't need. Personally, I purchased several different size ground bar kits for 2 or 3 different panel type, kept them in my truck. Became usefull for sub panels also for adding on to existing panels. Just my humble opinion, hope it helps.
~Ange