Looking out for Shemiras Einayim – How to deal with the scourge of pornography

Eid eched is belived even if he/she is a nogea bdaver because it it a gezaras hacasov. We see this idea from Rav Shimon ben Shatach who killed 80 witches in one day (because they were involved in sorcery). The friends/relatives of these witches hired two false witnesses to testify against his son and his son was killed by Beis Din because eidem zommim were not presented to prove these witnesses false. Bais Din knew that the charges were utterly false but had to kill his son because of the gezaras hakosov that two witnesses are always believed. So too, whenever there is an eid echad he is believed as long as he is a kosher eid even if he/she is biased. Another reason why a biased witnessed is believed by eisurim is that one of the major sources that one witness is believed is from the Torah verse which states that a woman who counts her clean days is believed even though she is biased.
Regarding the second riddle, since the giving of maaser is rabbinic, we can say the Rabonom made the rule uniform even though it may not apply in some limited cases.