Why can't Android phones record VoIP calls?

I use Republic Wireless so the majority of my voice calls are VoIP. I use a call recording app to help me recall conversations after the fact. But in order for it to work, I have to turn off wifi and make the call over the cell network. The recording app made me acknowledge that I understand that no app can record VoIP calls. Question: why is cell recording allowed, but not VoIP. Is it the OS limitation? Is it for security? Is it a legal reason?