Hello,
After research, I’ve found that is by design. The cause of your issue is that you didn’t grand correct permissions to users. In fact, only users with permission level equal to “Design” or even higher can delete content of Forms folder. So, what you should do is to assign only “contribute” permission level to those users. Thus, though they can view the forms folder, they are not able to delete it.
Regards,
Jerry
Xing-Bing Yu