While reposting is wrong, some of their excuses aren't completely untrue.
"I didn't know." -- Many people who repost something don't realise that they are reposting. In many cases, it's because the new post came weeks, months, sometimes even years later, or sometimes they are completely oblivious.
"Everything's a repost. It's impossible to make something original nowadays." -- While it is not impossible, it is often quite difficult to create something original. There are so many memes out there, most often, we aren't the first ones to have this idea.
Reposting is not okay, but it is okay to cut people a little slack sometimes.