It's an interesting question, right? The blue line seems like desecration. The road grime seems like desecration. The cow piss and shit sure smells like desecration. I'm not a huge flag guy but I've always been careful with it since veterans can get upset about treating it rudely.

I think the intent of the person needs to be considered.  I strongly suspect that the trucker believes they are respecting the flag and what they believe it stands for.

On the other hand, the whole blue line thing is pro-cops, right?  So maybe it's just pragmatic and a "please don't give me a ticket" kind of thing.  Maybe it's a minority who has had a lot of bad experiences with cops and they are hoping this makes life easier.  It's a time of year where keeping a vehicle clean is difficult.