I get this question sometimes—on the range or via email. The answer is because competitors keep pushing the envelope. Almost 90% of the DQ and procedural rules, along with the equipment rules, have come about due to competitor actions.
Case in point: I received a question via rules@uspsa.org concerning a competitor who shot a port open then proceeded to shoot the now-available target through the open port. Some may argue that the WSB should have been specific, but who shoots props deliberately? What’s next, shooting the stomp pad? The stick holding up the swinger? Actions like this should seriously warrant a DQ, simply for destroying club property, and are the reason that the rule book continues to grow.
Another example: a competitor has been competing in PCC division with a brace and foregrip combination that does nothing but contain a handgun, thereby violating 5.1.10 by adding a stock (brace actually, which isn’t legal either) and a foregrip to a pistol. It does not replace the top end with another barrel or anything of the sort. Not legal for PCC. Even if you have an ATF SBR stamp, this only means it meets the ATF rules, not USPSA rules. Legal conversion kits normally replace the entire top end, using the frame and fire control system but not the original slide and barrel. Not following the rules or not inquiring if they aren’t clear just creates issues that grow with time. Take the time to review the rules and if you aren’t sure, ASK. We are always willing to help.