Isn't this endemic in beaches throught the country? I grew up in NY and one summer job I had was testing beaches for bacteria levels. Most beaches were one rainy day away from being closed down, and they were certainly nowhere near Mexico.
I think this is the most salient point -- most beaches are probably fine most of the time, but when you get a big storm or some other event that causes a lot of runoff, the beaches quickly become health hazards. And it doesn't even need to be anything particularly nefarious, even just dumping a bunch of nutrients into the water will cause more things to grow, nasty things included.
I've been to plenty of beaches where after a strong rain, they had signs out on the beach warning about potential sewage runoff, and to avoid swimming in the water.