Can a professor force students to pay a subscription fee for a different site in order to take classes?
This started as a comment but I decided to give an opposing view from the one in Anonymus Physicist's answer.
As you might have guessed, the answer to this question is location dependant.
In my University in the UK, this is explicitly forbidden. It is, in fact, forbidden to ask students to register to any website or service in order to follow the course materials, even if the registration is free. There is a (fairly short) list of approved services, for which the University has ensured that they follow the (now local, but still based on GDPR) Privacy Laws, which students may be asked to use. This list is updated yearly, and I believe the process is substantially more complex than sending somebody an email saying &quot;I want to use X in my classes next year&quot;.
As this is done to ensure compliance with Privacy Laws, I am fairly certain that all British Universities have the same clause. As the current Privacy Laws are still based on GDPR, originally passed as European Union legislation, I would guess that something similar is in effect through EU Universities as well.
(However, note that since UK Universities are ran much more like companies than public education institutions, as is common on the Continent, they are also much much more careful about any legal liability their actions might cause. So it might be less enforced outside of the UK.)