Interference in the U.S. is almost always the result of military testing or training. If you check against another map (Google maps, or skyvector.com/globe.adsbexchange.com to see sectional charts) you'll see that the interference region is usually centered on or close to a military base or Military Operations Area. There are often FAA notices to pilots about potential GPS outages in those areas.
Around Syria, C4ADS did some research a few years back identifying Russia as the main culprit; I think the conclusion was that Russia is jamming GPS to defend their bases from drone attack. https://www.c4reports.org/aboveusonlystars
At least some of the jamming around Moscow is thought to be an anti-drone measure (to stop nosy photographers? assassins?): https://www.gpsworld.com/jammers-at-dachas-add-to-russias-ab...
In the Baltic, the assumption seems to be that it's usually Russian military.
I wish someone more knowledgeable about what's going on in other regions with interference would write about them! I just don't have the time and expertise do dig into all of them.