While some companies ban cell phones for security reasons, others prohibit them for productivity reasons.
We are there to work, not talk on the phone or text. If there's an emergency, then your spouse, family, or child, school, etc. can call the office phone.
The smart watches are simply a way to circumvent the rules. Therefore also being prohibited.
I've worked for places where you had to lock your phone up at the beginning of the day. And you could use it on breaks, and during your lunch hour. If you were found to have it with you at any other time, then you were fired on the spot.
It's not always about security. Often it's just about the employer not paying you to waste time on personal matters. And if the business has an office phone, then there's no reason you need a personal phone while you're working.
Yes we all like to be able to handle little personal things quickly on the sly. But it's not what we're being paid to do.
Since smart watches could talk with a phone that's hidden, it makes total sense that smart watches are being prohibited.