BERLIN—When Amsterdam pioneered “coffee shops” in the 1970s, the European capital was one of the only destinations where you could openly buy and smoke weed—it quickly became a global mecca for marijuana enthusiasts. But over the last decade, it seems the grass has grown greener on the other side of the Atlantic, with Colorado and Washington state legalizing recreational cannabis use in 2012 and Uruguay becoming the first country to legalize it the year after, followed by...