However, with pension funds this is slightly different. Funds have a huge voice and customers switching investments can influence how they use their votes. It's very easy to do, so why not do this in addition to all the other things?
It's also less about removing funds from fossil fuels and more about putting your retirement savings to better work. If you move your fund to a "future world" style one then this provides capital to clean energy projects etc. The ethical funds I looked at even performed better historically than the standard ones, so it's win-win.