Because they're dicks.
I'm sure you already arrived at that conclusion, but just didn't want to say it. They have nothing to gain by it except blowing up their own egos by thinking, "hey, if that God does exist (which he totally doesn't), IN HIS FACE!" In the Christian worldview, they have been deceived to the point where they feel the need to blaspheme a God they don't even believe is there in order to attempt to deceive others. They are wolves who don't even bother to get dressed in the sheep's clothing in the morning.
And I've never seen these videos. What exactly do they say? I find it hard to believe that an atheist can honestly commit blasphemy of the Holy Spirit which, if I understand it correctly, refers to attributing the miraculous works of Christ (or the Holy Spirit) to the demonic.