And I gotta say, I feel his pain. When I was first going through the rules, I couldn't figure it out either. Basically, "tool proficiency" is a skill by another name, why not just put it in the skill list? I still haven't found a good answer. One blog I found suggested that they wanted a mechanic that would require the presence of the thieves' tools to pick locks, without adding the mechanical complexity of a +2 circumstance bonus for "having the required tools."
Seriously? XD Why go through all that confusion? Why not just include good ol' Disable Device (or even the anemic 4E "Burglary" skill), and say that using it without tools gives you Disadvantage? Done and done, without adding a whole new and confusing mechanic.
-The Gneech :cool:
 Proficiency bonus +2, doubled for expertise to +4, +3 Dex bonus.