Mocano,
I reviewed OSHA and NEC and didn't see anything that defined "secure". I think you're going to have to make a judgement call as to what you believe is secure. What would the effect be if one of the tie wraps broke or the outlet slipped down the 2x4 to the floor? If a hazard or potential hazard is created you may need a better method of securing the devices (wood screws) to the 2x4.
MikeCJ