Ah, indeed, I had previously read the first half of this (which is from the API docs page):
It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.
... which made me think that this must be something special I didn't know about, but the second half, while surprising, seems to clearly indicate that in this case the redundancy is permitted.
I apologize to OP, I thought you were asking why there were two, as opposed to one, backslashes. Now I realize that actually, you were asking why there were any at all