IIRC, the sign verbatim says "Target shooting across roadway is prohibited" (will double-check tomorrow when at my 600yd BLM spot).

Most - but not all - BLM road in my area that aren't fire roads behind heavy gates made of steel railroad (easy bypass on dirt bike/foot/horseback, though) are pretty well maintained.

Shit, around Larch Mountain (Oregon, not WA), the Bull Run Reservoir that feeds Portland's water supply is situated scores of miles through thick, harsh forested wilderness areas, and because of this, the local BLM roads are gravel, but the Bull Run Reservoir access road(s) and adjacent series of fire roads are all blacktop.

Anyway, I hope nothing comes of it. Fucking absurd they are to have even bothered you in the first place!