Because MSNBC is an American organization and their coverage is American-focused, their bias relative to American politics is what’s relevant here. It doesn’t matter what their beliefs or policy positions are relative to any particular standard, what matters is whether or not their work presents the news accurately or in a way intended to mislead or influence their viewers in favor of one side or the other, which they clearly do. We don’t even need to agree on whether the Democrats are a ‘real’ left party, only that they’re to the left of the alternative and that MSNBC favors them.