R77 Can you give a citation on that, please?
The section of the PHB/SRD dealing with damage rolls just says "When attacking with a weapon, you add your ability modifier - the same modifier used for the attack roll - to the damage." with no mention of requiring proficiency to make that happen and the section on Weapon Proficiency says that "Proficiency with a weapon allows you to add your proficiency bonus to the attack roll for any attack you make with that weapon. If you make an attack roll using a weapon with which you lack proficiency, you do not add your proficiency bonus to the attack roll." Neither mention proficiency affecting the damage roll.
Unless I see a citation that said otherwise, I would rule that you add your strength modifier to the damage roll for an improvised weapon because it fits the damage roll rule I quoted. (It also passes the logic test: it's hard to imagine that getting hit with a chair by a 300 lb. mountain of muscle wouldn't deal more damage than being hit with the same chair by a frail old man who can barely lift it.)