Possible lawsuits. The company that makes GVmate does not seem to have any other product. Obihai has a business to protect beyond connecting to Google Voice. Ultimately PcPhonesoft the makers of GVmate have nothing to lose by violating the terms of service. They are also a much smaller target with far less users.
Now that it has been made clear by Google that the service is not to be used in this fashion it's important to remember that when using GVmate you are also violating Google terms of service. If Google should ever decide to crack down on that you could potentially lose your account. That is not something I expect to happen but it is something to consider.