I think some people just think it's unlikely. At least the camp that I'm in. Certainly not offended by it, and it's something that is fair to discuss.
To me, the idea that a 13 year old girl from a blue collar family in a small town would be sophisticated enough to have a second cell phone is a little far-fetched.
It would shock me even more if she was in contact with a boyfriend/pedophile/etc. and the FBI couldn't find that connection.
That being said, it's certainly possible. Likely? No.
I put it in the same category as any theory that involves Jayme being a participant in her parents murders -- there is zero evidence to support it, but it can't be 100% ruled out.