This practice is not legal, but this is not a deterrent. It is just like a smoking device designed for smoking tobacco only, yet you know very well that they are not used for tobacco smoking but for other illegal substances. These apps are sold to people with disclaimers that they are not to be used for illegal purposes.
If you are unsure as a user of this spy app, do not say you are not warned. No matter how the product is advertised, it is against both most state and federal law to listen in or have access to communication by another person without their consent. The United States does not take kindly to cases of violation of privacy, and it could easily land you in prison for a long time.
It is not easy to know if your device is being tapped. There are, however, some signs that you could look out for. First, be on the lookout for noises and clicks that may be out of place during a call. Also, if your device lights up even when you are not using it, there could be software running in the background.
To remove these and stop the mobile spy, restore factory settings. This wipes the entire phone clean. Although you lose all your data, if it is backed up then you can just as easily restore it afterwards.