I'd never thought about it before, but that name is something of a misnomer. The Agency isn't enforcing drugs (hopefully!). They're enforcing (anti-)drug laws.
Strictly it should be the Drug-Law Enforcement Agency, I suppose.
[Edit]: I just checked: the name of the US DEA is actually the Drug Enforcement Administration, which is just as much a misnomer! One of its predecessors had the rather more logical name 'Office of Drug Abuse Law Enforcement'.