The landscape orientation of mobile phone only works on one side of the landscape.
So I suggest to also enable the game to rotate on the other side.
You may say this suggestion is not really important, but here are the reasons that may change your mind:
1.) Convenience. When earphones/charger were connected to your phone while playing, it becomes a disturbance (especially when you were holding your phone or using your thumb on using the virtual controls), so you tend to rotate your phone to eliminate this such inconvenience.
2.) Looks More Professional. Almost all landscape smartphone games out there have this “minor” feature, so it means that developers didn’t include this for their games just for nothing.
3.) Efficiency. One aspect I learn in order to become a great developer is to consider and expect all possible situations and problems of the users. So I consider phones with dead spots/pixel zones on the touchscreen, which may make your game unplayable, just because a certain button cannot be pressed. And the common solution for this problem is to simply rotate your phone to avoid the “not working touch zones” of some phones.
I hope you understand the importance of my suggestion. 
Thank you for reading.
P.S. Images are not mine.