Mmh. Cannot reproduce the rationale. Was overwhelmed by having no planning tools. Planning tools would have helped not being overwhelmed.

Anyway, if they do want to lock it, they should lock the hotkey as well. In the current state it is a bug, no ifs, no buts.
... Incidentally it's also a code ...

Blueprints can be created and placed by Alt+B right from the start of the game. Using the GUI to access the same functionality is, however, locked behind this misleading mouse-over message:
"This shortcut requires the Construction robotics technology."

Well, it is technically correct, "from a ...

Abstract:
Add a sign "end of regulated traffic" to the technology Rail signals .

Works: as an always-green signal, performing no checks, taking no signal inputs
Looks: single tile signpost with yellow triangle sign, no connection to the track
Usage: place instead of a normal signal at the ...