There have been restrictions in what you can add to your Facebook cover photo and what you cannot. It is indeed necessary to put restrictions else adding what I want means advertisement on Facebook for free of cost.

Major things that are NOT allowed on any Facebook Page cover photo are:

• Discount information on products
• Use of Like and Share visuals
• Contact information
• Content to encourage sharing

You can also read complete cover guidelines on Facebook.

Facebook is on the right track.