How do you know that they were locked by regular users? Regular users don't have that power (I believe) Usually you will be given a reason or it will be stated beside the locks such as [ans] [dup]
Have you asked a mod/admin about this on their profiles or anything? And xeano has a good point - if you supply links we might be able to give you a concrete answer :]
They weren't locked by normal users. The mod/admin just didn't post a reason about why they locked it or deleted it. It either turned into spam or it wasn't up to the standard they thought it should be.... or it violated terms and conditions... or it violated unwritten netiquette like a "I don't think women should have equal rights" thread I saw come up once. Does that clear it up for you?
dup means that there it was a duplicate thread... meaning that there is another highly similar thread that was around within the last few pages of threads.