Very importantly, it has traction, particularly with Oracle's big audience of administrators and system programmers from a proprietary Unix background. Even if SystemTap does absolutely everything DTrace does, you know DTrace already.
This isn't a _huge_ audience, but it is one that's comfortable paying Oracle money. Making sure these people have a reason to ask specifically for Oracle's Linux, not Red Hat, will pay for itself. Previously DTrace was a reason for Solaris administrators to resist going Linux at all, but Oracle doesn't care about that, so it wants DTrace on Linux.
The license doesn't actually matter so much. If you know you want DTrace, that's a reason to choose Oracle, because the DTrace experts work for Oracle.