I think it's BS. If FW rays did get into natural waters, the damage they "may" inflict would be much less than the sportsfish species the gov't has transplanted over the years. (ex: striped bass from the east coast). In fact, FW rays could be instrumental in keeping certain populations of invading species in check, such as mitten crabs, P. alleni and clarkii crayfish, and zebra mussels.
Stingrays are very good at eating inverts, and would be important allies in controlling the population of these invasive species, IMHO. Seems that not only the Brazilian government does this kind of thing...
Best regards.
Daniel.