One of the main principles of the Sikh faith is to Jap Naam. Along with earning an honest living and sharing with others.
Jap Naam is translated as reciting Gods name.
This Keep Calm… derivative goes well with the priciples of Sikhi / Sikhism as one of the 5 evils in Sikhism is anger. So by following the edict of Keep Calm and Jap Naam we are following the basic ideas of Sikhi.
Also it has a nice flow to it!