If you get a reply, they will say, glibly, something like "because they are effectively clumping in UK conditions" or "because it assists our customers to understand they are not difficult invasive plants like sasa palmata". And carry on. They may even call in their defence the TV personality gardeners who call them clumping. The real, unspoken, answers are "because we want to sell them, and we think that calling them 'running' would frighten off the customers", and "clumping is not a scientific term like pachymorphic, so I won't get done by trading standards for misusing it".

I'm not defending them. I think it's shabby.