Why "tax the rich" is the same demagoguery as always

Having lived first in the U.S. and now in Israel, I can attest that "tax the rich" is a cry heard in both countries, though probably more obsessively in the U.S. Thomas Sowell gives the most clear, concise explanation I've ever read as to why it was and will remain demagoguery. I also think it's immoral--if someone has honestly, legally, and legitimately made very large sums of money (that one can lie awake at night in envy over if one wishes), the government doesn't have a right to confiscate half or one-third of it.