The sentence isn't wrong - it's just a little awkward. Don't try and make sense of 'as for' - in fact, the 'as' and the 'for' aren't connected. The sentence can best be understood if you consider the three words 'apply' 'as' and 'for' separately: There is no preposition attached to the verb 'apply'. The core of the statement is simply 'The same rules apply.' The word 'as' has the meaning of 'like' - it's part of the fixed phrase 'the same x as x' The phrase 'for the traditional letter' can be understood as a ellipsis of 'the guidelines for the traditional letter.' So, the sentence is a shortened version of saying. [The SAME guidelines apply] AS [ [the guidelines] for the traditional letter] I hope that makes sense.

The sentence is wrong. It is indeed, as you mentioned, "apply to". "as for" is more commonly used to continue on a previous subject or sentence. Ex."As for you, I will deal with you later."