why this is illegal activity?

It is quite possible that in a system two data stores are sharing or synchronizing data with each other, in this case one data store will be communicating directly with the other; then why this is illegal activity?

Yes. It is quite possible that in a system two data stores are sharing or synchronizing data with each other. But this sharing/communication is done through a certain process. Without a process this can not be done. Lets map this scenario in real life. For example, there are two libraries. One is HEC library and other is vu library. Lets say they both represent data store. Data is being communicated between these two libraries. But this communication involves some process for sending, copying or recieving data. Libraries itself can not do it. So it is illegal to show direct communication between two data stores. There must be some process for data commincation.