Filters drive me batty (and are why I'm so glad the school I work for doesn't use them. Period.)

I can think of a few reasons that aren't the librarian's fault - it may be pressure for a specific filter from the local community/library board (there are parts of Colorado that are pretty fundamentalist/etc.)

The other is that the cheaper filters tend to filter more, and less intelligently. If there's a mandate from the town/county/state to filter (as is true for a lot of government funds for 'Net connectivity), they may not realise how bad it is, or be locked into a contract for X months more that has them with a bad filter.

(There are many more reasons I've got issues with filters, but the fact that can happen is a major peeve.)