Sorry about that. I'm actually in Tn.
So y'all think it would be legal for them to force employees to do this?
If a person didn't have a cell phone, then they couldn't be forced to do anything to said phone. Wink wink.
Personally, if an employer wanted unfettered access to my personal phone, they could take a long walk off a short pier. If someone wants me to carry a device they control, they can provide that device and pay the bill for it; otherwise, I'll seek employment elsewhere.
And, really, that's the rub isn't it? You can refuse any demand your employer makes; there's no way they can force you to do anything. Just be prepared to reenter the job market after your refusal.
What is the purpose of the app?
Just a wild guess out of left field here, but Salesforce is the primary provider of customized apps for employees. Personally, I've never liked their apps (nor their CEO) and I was really sad to see ExactTarget swallowed up by them.