Part of my job is to advise companies on security policies like this, and I have advised in favor of such restrictions when asked. However this is done out of respect for the end-user's privacy. The reasoning is that there are two conflicting priorities in permitting BYOD use and network access:
First, as a security officer I have a duty to ensure that the network and all devices connected to it remain secure.
Second, as an agent of the company I have absolutely no right to dictate to an employee what they must or must not do with their device to prove that it is secure. It is their device which they purchased with their money to use for their own purposes.
Since I cannot prove that the device is secure without violating their privacy or exerting an unreasonable amount of control over the device, the only resolution is that the device is not permitted.
If you really need a device, then the resolution to that is to get the company to buy you a device -- at which point the company owns it, and can dictate what security measures are taken.
At the end of the day, a company pays you to do a job, and as such has the final say over how you do it and what tools you use to do it. It may not be your choice, or the best choice, or even an efficient choice. But that's how they want it done.
Good employers will listen to their staff and make adjustments and get the tools that their staff need. But it isn't mandatory.
If you don't like the job, and the employer won't change it to suit you, you have two choices: live with it, or leave.