The "rule" that says:
For, when used as 'because', can't be placed at the head of the sentence.
is nothing of the sort. This is not now the most common form, but such sentences are perfectly valid, as is the example sentence, where "for" does indeed mean "because". It could also be replaced by "Since" as another answer points out.
Take these hands and lift them up. /
For I have not the strength to praise You near enough. /
For I have nothing, I have nothing without You.
(Bebo Norman – "Nothing Without You")
For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
(Romans 1:16, KJV)
For I am convinced that neither death nor life, neither angels nor demons,[a] neither the present nor the future, nor any powers, 39 neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.
(Romans 8:38-39, NIV)