Different grades of plastic. I think the stuff the cups are made out of isn't as stable as the straws. Still better with paper but its worth pointing out since they don't degrade or pose the same risks.
Most fast foot places I've seen use those wax cups. I'd love to see a return to glass. Even at grocery stores. Have a BYOG policy where if I want cereal or drinks then I can bring a big jar and fill it myself.
The triangle like symbol on the plastics will tell you which type it is