That'd be establishment of a business not establishment of citizenship.
The 1989 FTA recognized that a hospitable and secure investment climate was indispensable if the two countries were to achieve the full benefits of reducing barriers to trade in goods and services. The agreement established a mutually beneficial framework of investment principles sensitive to the national interests of both countries, with the objective of assuring that investment flowed freely between the two countries and that investors were treated in a fair and equitable manner.
See? Why would they say investors from America/Canada need to be treated fairly if we were all suddenly citizens of the same entity. We'd all be treated the same. No need for this statement at all.
Thanks.
National treatment: The principle of providing foreign producers and sellers the same treatment provided to domestic firms. For example - and I believe that this was an actual source of contention of the national treatment principle - when refining gasoline for domestic uses around the U.S., many regions have their own requirement for additives and blends. What you probably did not know is that some of these blends have been formulated so that certain agricultural products (like corn) have to be used in the refining process, thereby creating more of a demand for them. When a country like Mexico can refine gasoline more cheaply (labor savings and environmental standards relaxed) and export it to the United States, and as long as they refine it so that it meets the stringent requirement of an acceptable used blend for at least one region, then the corn producers, for instance, cannot lobby congress to have the imported gasoline stopped (which would have protected the artificial demand for their corn output if successful). National treatment does not allow for a country to discriminate against goods for import purposes, however, it does not preclude the consumer from discriminating against the good. But, in situations where the good is indistinguishable from another (natural resource and goods found in pure competition markets), the consumer seldom knows the country of origin.
I hope you will not continue to use the term incorrectly now that you've been properly informed about its meaning.