Now, I will say that I think any employer should be able to access the texting/phone records of communication devices that it lends out to its employees. The employer owns those phones and pays for them. To use the example given before that if it was your buddy's phone, well then the buddy most definitely should be able to access what was sent over his phone. I'm not sure what exactly the buddy could do to punish you for improper use of his phone, since he probably isn't employing you, but if you compare it to this case, it might be like suing him for telling your wife that you were using his phone to text your girlfriend.
And even if you assume that the employee is paying any overage charges, it doesn't mean that the overage, if there was any, was the only minutes that were used for personal purposes. And, from this case directly, it said that the guy only actually used the device 3 times while on duty to text for official purposes. So, that could be used to determine ways to buy the least expensive plan the employer actually needs. If official texting is only needed for a few times a month, per employee, then maybe the force could save money by paying per text, but they aren't going to know that without looking into the records.