The fact the ALA endorses such an idea should be more than enough for you.
And just because you have never seen it does not mean it does not happen.
Please show me where the ALA endorses kids being freely allowed to view porn in libraries. Perhaps the ALA opposes filters, but I don't think they endorse allowing children to view pornography.
As I said earlier on the thread, I word for a school system which has filtering software installed - it doesn't discriminate between adults and children; it filters for all of us. Some objectionable sites still get through. In general, I think the software is a bit overzealous: some useful sites such as Free Republic are blocked. I don't think filtering software is a perfect solution. I think in general children should be accompanied by parents who are monitoring their behavior in any public place, including a library.
And just because you have never seen it does not mean it does not happen.
Do you have any evidence that it's happening with any great frequency?