The problem here is that when you want to use a photo, you either take it yourself, or you take one made by someone else and ask him what are his conditions.
When you take a photo and put it one your web site, this doesn't mean that anyone can grab it and use it.
The problem here is that people think that they can do what they want on the web and if you just send them a mail they will do nothing. DMCA is globally a bad thing but but its at this time the only way to make people respect copyright. It should go away but for a better law.
You can transpose this to software easily. If you put some of your code on your blog, you expect people to respect the law and so your copyright. If your code is under GPL for example, you don't want other peoples to take it and include it in their commercial product without respecting the GPL.
Look at all the work done by GPL violation and how its hard to make people correct their behavior when your are a now know organization. This men is alone trying to protect its right and the only thing that he can do which is effective is to fill a DMCA.
And remember that the copyright law is what make GPL possible...