Second, the question was not whether piping to shell is a good way of doing installations. The question was whether it's actually worse than the common method of offering up an untrusted downloadable executable.
Many (or most) consumer software downloads that you'll find for Mac or Windows don't meet any of the criteria you specified (which I agree are good criteria). But that wasn't the question: Is it actually worse than offering an untrusted executable? People don't seem to complain about those as much, but to me they seem equivalently bad.