Depends on the motivation behind it, I think. If the purpose is to make sure people don't starve for its own sake, then yes. If the purpose is to reduce the cost on society of letting them starve (in a "stitch in time saves nine" way), then no.
A tax rate for each bracket has to be picked. No matter what, you'll end up with something that can be construed as a moral judgement. If higher tax rates for higher income is legislation of one segment of society's version of morality, then so would be a flat tax.
Which ones? Most that I can think of can be approached from the point of view of getting rid of externalities in business and personal decision-making.
Perhaps, though like other issues, it can also be justified in terms of overall societal benefit, IMO.
In certain regards, but I see much more legislating of personal morality (e.g. same-sex marriage prohibitions, "blue" laws, etc.) from the right than from the left.