It is an urban legend. The Quebec Ministry of Health and Social Services and other ministries in Canada have set standards with respect to cyanotoxins. In order to reach those cyanotoxin thresholds, there must be very significant concentrations of cyanobacteria. It is important to remember that cyanobacteria occurs naturally in aquatic environments. It's simply a matter of the type and quantity of cyanobacteria that are present.
I would agree that our way of managing lakes and rivers is not environmentally friendly and fosters the proliferation of cyanobacteria. The laws and regulations have to be changed. Eliminating phosphorus from domestic detergents is one way of reducing a small proportion of cyanobacteria at very low cost.