This is part of the growing (though hardly new) phenomenon called the consumerization of IT. Employees have always brought their own new and cool devices into a company network, though in the past these tended to be either high-level management or technically sophisticated users. And back then, even if users brought their own devices to work, it was much harder to use them, as they still needed the company-approved software from IT.
But today, any employee could be bringing in the latest smartphone or tablet, and these devices are much more sophisticated and powerful than many past devices. In addition, the vast majority of your core company applications today are probably Web-based, which means that all a user needs to access them is a browser--no special company software needed.
This means that, not only is it much easier for employees to use their own devices to access company resources, it is much harder for IT shops to even know they are doing this, never mind come up with a way to stop it.