It is because they are basically doing 2 scans (and charging accordingly). One scan before the contrast, and then a second after the contrast. Generally, if nothing shows up on the 1st scan, nothing will show up on the post-contrast scan. The insurance company sees it as excessive to do the contrast scan until they know for sure there is something to look at. Lawyers have started working hard to go after places doing scans with contrast due to the impact the contrast can have on your kidneys. On that basis, I am sure the insurance companies see it as in their own best interest to limit how many people get MRIs with contrast that may not be needed.

Sure it is more expensive down the road if you have to go back and get both scans at a later date and it is inconvenient for you, but it is something the insurance companies can use to limit how many people get exposed to contrast (and any subsequent litigation that may erupt down the road). It is just one more way that allowing lawyers to run rampant with medical lawsuits is raising our costs and making good care harder to get.