Does the requirement for Turkish nationals to have a short-stay visa when receiving services violate the terms of the EU-Turkey Association Agreement?

Notes on the Opinion of AG Cruz Villalón of 14 April 2013 in the case of Demirkan C-221/11 (pdf) by Kees Groenendijk, emeritus Professor of Sociology of Law at the University of Nijmegen. Concerns whether Turkish recipients of services are required to have a short stay visa or whether the current EU visa requirement violates the standstill clause in the Additional Protocol to the Association Agreement with Turkey.