Let me present to you(because I can't be assed to spend stone and stamina to make and break milestones in game) a crappy diagram:

Wherein Diamonds are the milestones, and the arrows are where the road is.
If the hearthling was standing on the red circle, his line of sight to the previous milestone is blocked, and as such would be forced to walk towards it, even if the path itself is not blocked. If the hearthling was standing on the green circle, the next milestone could be built with no hitches.
What seven is complaining about is how line of sight to the previous milestone is required to build the next milestone, even if the path between milestones is already cleared. Presence of various clutter and/or making tight turns with roads in this manner can make roadmaking frustrating.
I'd also complain about it but when I mine I pretty much carve out the entire cave, and I tend to walk to gather shrooms, cavebulbs, kill bats, and get glimmers and whatnot anyway, so I'm not really too assed about making roads in caves.
Was a pain in the butt to make a road in the forest though.
EDIT: Tl;dr, the arrows aren't getting blocked. Its the hearthling and the previous signpost that are.
W3 to W6 - Blockland. Special mentions: New Avalon, Tarn of Hotdog, Red October.
W7 - Elvenia, til we all got lazy.
W8 - Cult of R'lyeh, til the deeps called us back.
W9 - Tartarus, til death/wizards came for the land itself.
W10 - ???, ruined and forgotten.