No lock-in in most of the BS/BA/MD programs. If the students change their minds in a year or two either because they don’t want to do medicine any more or they don’t like the undergrad experience, they can always opt out and transfer some where else.
Among the places mentioned above, Brown and Rice are practically impossible to get in for ORMs. The former selects only low to mid single digits out of a large cohort of 60+ and the latter has only 6 slots.