Why do Soldiers and Airmen insist on wearing their Boots and Utes out and about OCONUS?

Because we can

Ok, the Army duty uniform is a utility uniform (ACU or OCP). Aside from lunch, essential stops like getting gas, Soldiers are prohibited from hanging out somewhere, going to the mall etc in accordance with AR670-1. Not all OCONUS locales are the same. Down town Heidelberg, no issue. To and from work, standard. You live in one blob of America on the south end of town, and your duty location is across town. The DFAC is on a third kasserne. Your motorpool on a fourth kasserne. The PX and CIF on a fifth kasserne. The international kasserne for the NATO support unit is on yet another kasserne. I could have been at any of those at anytime during the day. TDY in France, yeah there was some weird looks, but that was the prescribed uniform. To and from the hotel and the conference...then out of you uniform as fast as possible before you get your carbonade and Belgian blonde on. TDY in Turkey or Greece, much different environment. Uniform restrictions were in effect as threat dictated.

I guess it depends on the environment and intel. We went down town in Germany a few times but that was back in the Cold War days and Bitburg was a pretty safe risk. I've made "supply runs" in many countries outside of base on deployments but we usually wore civvies to do it. I know we never blended in at ALL but we made the effort. No reason to add to the potential threat/danger that's already out there.
Do what your pay grade will let you get away with I guess.