This is true, but often the front is not as conducive to drinking due to design. In the Chawan above, it is very natural to sip from either side due to the way the lip is shaped showing clear intent of the artisan.

Often this is the case. Deishi often has the ornate thick to no glaze medallion at the top front, it would be awkward to sip right there, so instinctively one turns the chawan.

Of course these are two examples of where it would be naturally appropriate to turn the chawan to sip, I am sure there are many more.

By the same token, there are many examples where I am sure one could simply sip from the front. Then it becomes more of a personal choice, I choose to turn, especially with Hagi.