As stated, credentialing is a hospital-based policy.
But, the reason why you should do the scans even if you cannot bill for them is for patient care. Being able to do a quick, focused, limited ultrasound examination can do wonders for patient care. Being able to do an endovag US to show an IUP prevents the patient many times from needing an HCG level drawn, requiring a foley, leaving the department for an ultrasound, and then adding overall time to the patient stay.
THere are plenty of studies out there that show that EPs can not only successfully perform limited ultrasound examinations, but we also save a lot of time in doing so. I think that most EPs that do ultrasound will state that we are not trying to replace radiology, we are just trying to enhance patient care with what we are doing.