It's one of those weird game theory things. Your goal is to prevent dangerous liquids from being brought onto planes, but you can't reasonably tell them apart from other liquids, so you ban all liquids. Because it's banned, you can be reasonably sure that nobody is going to try to bring dangerous liquids through security in the first place. (Not because people always obey bans, but just because they know it'll be confiscated.) So you can be confident that all the confiscated liquids are not dangerous, and it's safe to just throw them in a bin. Yet you still need to confiscate them if you want to keep people from bringing the dangerous materials through.

If someone wants to blow up a security line, there are much better ways to do it than allowing their liquid explosive to be confiscated, so you can be pretty confident that nobody is going to attack the system that way either.