Is Arnold a liberal at heart or is it just for attention as you say? I mean the tide is turning and being liberal is not that cool anymore, he should adjust. I mean Arnold was the guy who said he admired Hitler, but who doesn't, Trump certainly does 
But who knows, maybe he really is liberal and on the left on social issues and everything else...
Arnold is whatever he needs to be to maintain power/relevance. For decades he courted the Republicans, hoping they'd change the laws so he could run for President. They basically told him it will never happen, so there is a spite there. The Democrats offered him a home, and the old guard there still prop him up as a manly-man, so he goes along lockstep with them.
Past few years, Stallone has far surpassed him.