Is it safe to fish?

Why do people choose to fish in area's where the water is clearly polluted? I have scene some people throw their pole in some pretty ha-nus rivers. Seeing this makes me wonder, do they catch and release or catch and eat....hmm. Seeing this sparks me to give people a few tips on when not to fish at certain spots.
1. When the area is not inhabitable for mosquitos...you might not want to fish there.
2. When you don't hear a frog...you might not want to fish there.
3. When the water doesn't move...you might not want to fish there.
4. If you don't see a elderly man trying to give advice on how to fish in a area....you might not want to fish there.