I think you need to make a distinction between initiative constitutional amendments/initiative statutes and legislative constitutional amendments, as well as referred measures. Like Prop 67, voting yes merely endorses the legislative position on the measure. It is a referred measure which means that enough signatures were coughed up by the plastic bag industry to go to the voters. Voting no would be tantamount to saying that 8% of the population who were sufficiently badgered to sign a piece of paper can veto legislation based by two houses which were elected democratically.
Same with Prop 58 — the legislature wants to neuter Proposition 227, but cannot do so without the consent of the electorate. Or with bonds like Prop 51, which by state law must go for voter approval.