The United States is very big. If you're from a smaller country (particularly if it's smaller east-to-west), it can be a little bit hard to comprehend how different the weather can be from one part of the country to another. While the weather does typically travel from West to East, it can change significantly along the way, and it usually takes several days to get from one coast to the other.

The highlighted area on the map is a massive region, wider than France and Germany put together (though much less populated). In fact, it's quite rare for even this much of the country to have the same weather pattern. The simplest answer to why trees to the east and west are safe is that it's not as cold there.

There are some other factors, too: just past the Western edge of the highlighted region are the Rocky Mountains, which significantly change weather patterns. The highlighted region consists of remarkably flat land (leveled by glacial action), meaning that there's not much to break the wind as it sucks away the heat from the trees. To the East if this highlighted region are the Great Lakes, which also change weather patterns.

But the biggest answer is, it's just not as cold there. Cleveland, OH (at a similar latitude, but further to the East) is going to be almost 20°F warmer than this (which is still bone-chilling, but not tree-exploding), and Boise, ID (similar latitude but to the West) is going to be almost 40°F warmer (practically tropical! /s).

It's also worth noting, if you're not familiar with the US map, that the city of Minneapolis (where the anti-ICE protests are happening right now) is right about where the bottom of the "R" in "TREE" is on this map.

Total guess, but: Lake effect probably prevents huge temperature swings further east (the great lakes are GIGANTIC sinks of energy), and there are mountains to the west, I believe. The problem is the flat area pictured.

Also, the shape of the polar vortex wind mass isn't uniform, so it likely dips further south in that region than others (in part due to the barriers described above)