As regulardoc pointed out hospitals used to risk stratify patience to determine if medical or surgical prophylaxsis was necessary; however, now it's an opt out protocol essentially meaning all patients will receive a px dose of an anti coagulant unless contraindicated. These protocols are what Lovenox reps spent their remaining years helpIng hospitals establish. This will be driven forward by CMS core measures (Medicare attempt to improve hospital patient care and quality) and for surgery something called SCIP.
Also hospitals will be rated http://www.hospitalcompare.hhs.gov. Not only is there a threat to lose a DRG from Medicare for a readmission but also the potential to lose business when compared to other hospitals and their rated quality care etc.