NOTES : Since VS Becuase ...

Yes, we often use "since" and "because" almost interchangeably nowadays. And, yes, this is wrong on the GMAT.

As you noted, "since" is specifically used as an indication of time. "Since 1996, the Internet has grown from X users to Y users."

"Because she was hungry, she ate all the pie."

It's not okay to say "Since she was hungry..." (at least, not on the GMAT). So if you see a split between "since" and "because" and it appears that they could both work, use "because." (You probably wouldn't make the mistake in the other direction - "Because 1996, the Internet has grown...")