Musescore Pro ($6.99/mo) is required to download sheet music from musescore.com recently. (However, a few months ago, it was free to download.)
Many musics on musescore.com are already in the Public Domain, that means either the author posted them in Public Domain, or the author has been dead for over 70 years. Do they need to pay to those composers who died hundreds of years ago? Update: sheets in Public Domain are able to be downloaded without Musescore Pro now, but we still need an account to access them.
Also, there are many sheet music authors on musescore.com who created their own songs and posted them under CC-BY-NC (Creative Commons Attribution-NonCommercial) License. Is it illegal that they sell them for profit? Note: Putting ads (to sell Musescore Pro) on the website also means that they use it to generate revenue.
This script is available as a Userscript. To use this Userscript, you need to first install a user script manager, like Tampermonkey.