I should have chosen my words more carefully. With a 28, you're an eligible applicant at any of the top-20 schools (with the exception of WashU).
It ought to be noted that there are plenty of allopathic med schools in the US (e.g., KU Med) that accept a signficant number of applicants with MCAT scores below 28. In fact, the average for matriculants at KU is a 27. At schools like KU--i.e, schools that accept significant numbers of applicants with MCAT scores close to the national mean--the AdComs rely heavily on assessments of non-academic attributes (interpersonal skills, compassion, etc.), which means that interviews at these institutions are critical.
So, to a limited extent, I agree that there is no rigid cutoff for acceptance to medical school in the US. This is certainly the case at unranked schools that emphasize non-academic attributes over numbers. HOWEVER, at more prestigious schools, numbers become increasingly important in the screening process.