"promote the general Welfare" did not mean what it does today when the constitution was written.
If a mass murderer calls killing homeless people "volunteering at the homeless shelter"; that doesn't mean you should too, or that real volunteering should be stopped, or that volunteering means killing people. Or that killing is legal.
To put it more plainly, some asshole politician naming wealth redistribution "welfare" more than a hundred years after the constitution was written, does not make it constitutional.
Plus, the preamble is not technically a part of the LAWS set by the constitution, only the explanation of the founder's principles and beliefs.