Yes, employee monitoring software is legal.
In the US, companies have full rights to their property, including computers and mobile devices. Furthermore, there is no federal law that requires US employers to inform the employees that they are being monitored. In some states, however, consent is required.
In the EU, it’s also legal, but employees must be notified that their online and communications activity is being monitored. However, when monitoring employees’ emails, IM, or keystrokes, you’ll need a written agreement.