Found this on wikipedia:
The first tier, called Regionals, consists of 16 locations that include four teams, seeded 1 through 4, competing in a double-elimination bracket. The 16 host sites are determined mostly by merit – most No. 1 seeds host – but are also contested by bids from schools guaranteeing the NCAA a certain amount of revenue from that regional.
https://en.wikipedia.org/wiki/NCAA_Division_I_Baseball_Championship
This makes it sound like they generally pick by merit, but teams can bid to be a host even if not seeded in the top 16. So it sounds like a school could "bid" to host if not a top 16 seed, but could be chosen as a host on merit without "bidding".