It's perfectly grammatical. It's mainly an informal sentence pattern, and so it's true that it's usually avoided in formal style—but that's not because it's ungrammatical. Grammaticality and formality are two very different things.
The basic pattern looks like this:
(just) because + clause + does not mean + that-clause
This is covered in The Cambridge Grammar of the English Language (CGEL) on page 731:
Because is the most central and versatile of the reason prepositions. A PP with because as head can occur in subject or predicative complement function as well as adjunct:
 [i] Because some body parts have already been turned into commodities does not mean that an increasing trade in kidneys is desirable.
 [ii] The reason I didn't call you was because the phone was out of order.
Because could be replaced by the fact that in [i], that in [ii], and these latter versions would be widely preferred in formal style. In the subject structure [i], because is often modified by just, and the matrix VP is more or less restricted to doesn't mean: Just because you're older than me doesn't mean you can order me around.
As you can see, this sort of PP can occur in subject position, although it's basically limited to this particular sentence pattern (as CGEL says, "the matrix VP is more or less restricted to doesn't mean").
One last note: grammar is a description of how native speakers use the language. Certain learners may be unwilling to recognize because-PPs in subject position, but if native speakers regularly use and accept them anyway, all this means is that the grammatical description used by those learners is inadequate.