I think because there's some nude pics at different agencies.. these filter consider micro website like that. At the same time these filters are also a way to censor what some "organisation" qualify to be against their own interest.
Lately here in Canada, a law passed throw the opposition who was suppose to read carefully each papers and now the liberty of making movies whit content that maybe can hurt the interrest of the "in place" government (conservator for now), can be censored by cutting the government funds to the creators.
Some of these filters are sometimes a way to control the overall people opinions. I think their managers don't know how much power they have in their hands or know it very well.