Because as a preposition

Conventionally because is considered a conjunction, because it links (or conjoins) two clauses. The argument against its classification as a conjunction is that because operate like other subordinating or coordinating conjunctions but does operate like prepositions.

This post on the Language Log demonstrates that because is not a conjunction by comparing it to that (subordinating conjunction) and and (coordinating conjunction). I summarize the argument below.

Typical subordinating conjunction: that

• That can be omitted from a sentence and the sentence's meaning generally remains the same.
• The clauses cannot be rearranged and maintain meaning: "[clause 1] that [clause 2]" != "that [clause 2], [clause 1]".
• Complements follow that.
• If because is omitted from a sentence the meaning does not remain the same.
• With because the clauses can be rearranged.
• Adjuncts generally follow because.

Typical coordinating conjunction: and

• The two clauses joined by and can be rearranged and the meaning stays the same.
• And cannot start a sentence.
• Rearranging the clauses changes the meaning with because.
• Because can start a sentence.

Because does not act like conjunctions, and thus is not a conjunction. It does, however, act like prepositions.

Prepositions can introduce noun-phrases, clauses, preposition phrases, and nothing, depending on the preposition. Because has long introduced clauses and preposition phrases (with of) and more recently also introduces noun-phrases.

It's quite correct, and has been accepted by the Authorities. The Academy has voted because into the Ancient and Honourable Order of Prepositions; because of thus becomes optional usage.

The preposition because was the American Dialect Society's 2013 Word of the Year.
Because English, after all.

This usage is probably an artifact of texting. 'Because' is still a conjunction, but people are eliding the hell out of their sentences because [they] [are] too busy.