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Basically that's what I thought. I just checked the Michigan State University (grew up in Michigan) which has their tenured positions listed in a PDF file http://www.reg.msu.edu/read/ucc/Updated/07faclty.pdf . As I was scrolling down the pages the College of Arts and Letters is headed by Frank Rutledge, M.S. and there are a lot of other full professors and assistant professors with M.S. and M.F.A. degrees. This department seems to be the exception. In the School of Journalism one tenured professor James Thomas Detjen only has an M.A. and he's the John S. and James L. Knight Chair in Environmental Reporting. The College of Social Science is headed by Theodore Curry, M.B.A. There is also tenured Professor with an M.S.. The College of Arts and Letters Department of Theater has an acting chair is an M.A.

These are all exceptions, but someone chairing a department without a PhD does happen.