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Cannabis is semi-legal in Spain via Cannabis clubs.
However wikipedia suggests "Cannabis plants that are located somewhere visible from the street/public place (ie. from balconies) are considered a serious administrative offense, which leads to a fine from 601 to 30,000€"
I guess the plants aren't visible from the street, but because a helicopter picked them up and were broadcast to the world the police were probably forced to act.
Agree: very hard to spot they are marijuana plants from the telly box, but I am guessing the position and density of the plants would have triggered alarms. Guessing growing weed on roofs is a thing in the town/country?
In this country the police look out for house where the snow melts on the roof.