7z is not a proprietary format. It is an open format, the specification is open for anyone to evaluate, and software makers do not have to pay any fees to implement the compression and decompression algorithm.
Zip is based on the deflate algorithm and is also free to implement, but has proprietary extensions that may be licensed from PKWARE and/or WinZip. Gzip, also based on Deflate, is an open format and is used as a free alternative to Zip.
RAR is a proprietary format, and software makers must pay a license to Rarlabs to implement the compression algorithm. They have allowed decompression without a license.