It turns out that it's possible for the server to detect whether it is running via "| bash" or if it's just being downloaded. Inspecting it via download and then running that specific download is safer than sending it directly to bash, even if you download it and inspect it before redownloading it and piping it to a shell.
The server can also put malware in the .tar.gz. Are you really checking all the files in there, even the binaries? If you don't what's the point of checking only the install script?