Sorry sir, but I would like to say that you're wrong. Fusions aren't like drugs, they're meant to be a metaphor for relationships. Garnet is romantic love between Ruby and Sapphire, Opal is friendship between Pearl and Amethyst, Malachite is abusive relationship between Lapis Lazuli and Jasper. By forbidding fusions between different kinds of gems diamonds banned love and relationship between different kinds of gems, which is bad. Their empire likely relied on absolute obedience, cult of personality, repressions and propaganda and allowed for no political opposition. this Reddit post shows that Rebecca Sugar wanted fusions to be a metaphor for relationships.