Why is it? It is normal for a program to lock files that it is
reading. The only programs which don't do that are those which read in
all of the file when it is first opened (like Notepad).
You can't prevent it from happening. You may be able to use tools to
force closed files open by other clients, but be aware that this may
well lead them to horrible errors, crashes etc.
You could consider using an intranet instead, a more robust way to
Adobe should just provide a switch, many company's share pdfs on a network drive that is read only to all but a few. In our work place most staff don't have frequent access to e-mail, so we have to walk around and find out who has a document open, ya, provide a switch it won't kill ya!
What are you babbling about? Ever heard of version control? Imagine that if every versioned file would be locked by some program. Even the source code for some running application can be altered on the fly and you are trying to convince us that locking files while reading(!!) is normal... hilarious. It's a bug not a feature, that's all.
What is the point of resurrecting this 5 year old topic?