Check what the UK specs are...they're allowed to tow and with smaller motors than what you have (2.3l). My feeling is that the NA Mazda lawyers say no towing for legal reasons. If the person lost control or had the car overloaded, they might try to blame Mazda in some fashion.
I believe that 1500 would be your max. If you do plan on towing often with your 3, I would suggest investing in an ATX cooler as a safeguard. The suspension should be fine.