They are just different games. USPSA does not incorporate the 'tactical' style of IDPA - so there are no rules regarding cover, cover garments, dropping loaded magazines, target engagement rules, etc. However, calculating your scores in USPSA is more complicated.
Ha! Yeah, I totally get it. There are a lot of guys with strong opinions about their game of choice. I've shot both, and both are fun in different ways... but only USPSA has Open