Who’s the culprit?

On the Gardener’s World forum someone recently asked for the ID of the Lamium galeobdolon Variegatum.

“This species is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild” as per http://www.plantlife.org.uk/uk/discover-wild-plants-nature/plant-fungi-species/variegated-yellow-archangel.

And yet it is available for sale, e.g. on the Telegraph Garden shop at https://gardenshop.telegraph.co.uk/lamium-galeobdolon-variegatum/prod25002. 🙄

Who is the most guilty, then: the innocent gardener who purchases this plant or the Garden Centre who should know better?