I guess they still have 1 million plates in some storage facility. Nah, jokes aside: what they actually do is to not only legally, but practically issue their own plates, which would include stamping them. Also, they don't use the reflective background white. So not only they don't need it, they also have reasons to continue with the old variant. (And actually, they might indeed have a huge stock of them. Military supplies are often aquired at ridiculous numbers.)

(PS: Sorry, auto-correct inserted a typo, just fixed)