Installing any kind of application to someone else's cell phone, including Mobistealth, without their knowledge and consent is illegal. Use and purchase of Mobistealth by itself is not illegal. In other words, the product itself is not illegal. It is what you do with it that determines legality. Whether or not Mobistealth is legal to use for the purposes of spying on someone else's cell phone is one of those laws that would be extremely hard to get an arrest warrant for let alone a conviction. How could you prove to the police a cell phone spy was installed? How could you prove who installed it? It is an issue that the police would most likely only pursue by placing a call to the suspect and warn them to "leave this person alone and stop harassing them."
You should know that when purchasing Mobistealth they are aware of these laws so if you come right out and admit to anyone in the Mobistealth staff you are going to spy on someone else's mobile device they might not sell you the product. The assumption is that you are going to use the Mobistealth spy product to track and monitor YOUR OWN cell phone.