The outside of a piece of meat comes in contact with a lot of bacteria making its way from slaughterhouse to table. Usually cooking a steak on the outside will kill all that off. The center of a single cut of steak is sterile, that's why you can eat it rare. But glued pieces of meat could contain bacteria like E. coli on the inside. 
Meat Glue is the colloquial name for transglutaminase, an enzyme that is applied to make pieces of meat stick together, for the purpose of deceiving customers into believing that the result is a single piece of meat.