pots of weeds

I had assumed these weeds next door were buddleja and were growing directly on the wall but when I asked the builder who was working up there to remove them he told me they were in a pot. I used the camera to zoom in to identify them and I see they are smooth sow thistle. Why someone would want those in a pot on a "balcony" (don't know what else you call this feature), I've no idea.