The primary reason for this is for your security of your members. Not being able to post a link prevents members from posting unsafe/ spam / or otherwise distracting links to your course. This way, your members stay focused on your content instead of being directed to an outside site.
As an admin, you can create course-related/important links in Website > Navigation section and add it to your site menu.
You can also create a Link List specific for the product and add it in Products > Appearance > Customize > Header