The joining-comma rule mentions that two complete clauses may be joined with a comma if the clauses are short or the comma is followed by a coordinating conjunction. So, I suppose that I should write
When I arrived home I fell asleep, for I was exhausted.
When I arrived home I fell asleep because I was exhausted.
However, what puzzles me is that "for" does have a causal (subordinate) role. Here, the logic of the language escapes me. Please, can someone clarify this point?