Well by default the script looks to see if you have node already installed and if it "looks ok" (ie responds to node -v with a suitable number) then the script leaves it alone to save time - and we don't want to force a new install on folk who may have a stable working version. It can't tell (afaik) that the sd card has just been plugged into a new box as it doesn't have any history.
Of course there may indeed be ways round these - but whether they are viable or not would need investigation for what is a rather small edge case. Especially when the actual issue is due to the underlying OS and node.js itself and not Node-RED per se.
The other thing to bear in mind (going forwards) is that Arm6 is getting less and less support - the only nodejs builds currently are unofficial ones - https://unofficial-builds.nodejs.org/ so while we do our best to make it look easy sometimes you'll have to work it out yourself.
If it's really a big problem then I'm sure we would be happy to accept a pull request if someone spares the time to investigate.
