On musescore.com, there are several song arrangements released under CC-BY-NC (or CC-BY-NC-SA or CC-BY-NC-ND) licenses. However, musescore.com won't let you download many of these unless you have a Pro account with them (which costs money). This seems to me like a clear and blatant violation of the "NC" terms of those licenses. Is this actually the case, or is it legal somehow?
If you read their user agreement, you will see that the content which is available to those who have a subscription may be owned by the company, or may be owned by users who upload such content. They allow content owners to specify license terms. §7 of the agreement is where the uploader licenses material to the company:
you hereby grant to Musescore a limited license to use, modify, delete from, add to, publicly perform, publicly display, reproduce, and distribute such User Generated Content solely on, through or in connection with the Service, including, without limitation, through the Service to applications, widgets, websites or mobile, desktop, including, without limitation, distributing part or all of the Service and any User Generated Content included therein, in any media formats and through any media channels, including without limitation, third party social media and streaming platforms, and shall constitute a waiver of any rights, ''moral rights,'' or any similar rights under any jurisdiction. Except as expressly provided for herein, this limited license does not grant Musescore the right to sell or otherwise distribute your User Generated Content outside of the Service.
Nothing in the license that uploaders grant to the company prohibits using the work in a commercial setting. Nothing prohibits a user from setting the license for their work as plain CC-BY or even CC0.