If you are a host of a venue, your team isn't allowed to play there. Either a school or a conference needs to host each venue. If a conference hosts, any school can play there still, but if it's a school, then that particular team must be sent somewhere else.
For example, the Big 12 hosted the Final Four in Dallas last spring. Marquette was the host school for the Milwaukee site that you mentioned. And I believe the MVC was the host for the St. Louis site, too.
UT-SA was the host school for the San Antonio site this spring that we played at.
If Iowa State is the host for Des Moines, then we wouldn't be able to play there.