It's not really socialist. Socialism is an economic model that involves taxing the rich and redistribution of wealth to the working class through welfare programs.
But in ST, there is no economy, no taxes, no rich people, no wealth, no working class. The only thing from that definition that they do have is welfare, but it's a completely different form of it.
ST is a magical post-scarcity utopia. Any economist would tell you that economics is first and foremost the study of how to allocate scarce resources. In a post-scarcity society, the whole concept of economics breaks down. Replicators break everything we know about economics. Everyone can get everything they need and it costs them nothing but electricity (which they conveniently can generate for basically no cost).