FTPS, like FTP, has 2 connections. One is the control, the other is the data. You initially connect over the control, and then when you need to transfer data(which includes everything from transferring files to listing directories), the control channel negotiates the parameters for the data channel. In that control packet, is the IP address of the server. In other words, it doesn't just connect to the same IP the control connected to, it actually connects to the IP address and port in the packet. With FTP aware devices like a firewall or LTM with FTP profile, the device rewrites the control packet to replace the IP address with the proper internal addresses to make everyone happy. Being that this is FTPS and the LTM cannot decrypt the control packet and make changes to it, the backend server needs to believe it has the VIP address to create a proper control packet that the client can connect to.
I believe this is only the case with Passive connections. With Active mode, the IP address of the client is in the control packet, thus probably making a SNAT setup break.