I would love to find any kind of proof that manufacturers were concerned that we as consumers were going to be forced to buy another charger with our new phone. Frankly, if the phone didn't come with a charger and the instructions said "You can use ANY USB charger from ANY manufacturer", then I would buy your implication as fact.
Also, to suggest that competing phone manufacturers would "agree" to do something in unison to allow their competitors' chargers to work with their phones or vice versa is as hard to believe at best.
The reasons they individually (not agreeing together) decided to use the MicroUSB standard are because they are more durable and have a much higher connect/disconnect lifespan than the former standard, MiniUSB, and also because they give the phone immediate and high speed access to data on PCs. There was no corroboration between manufacturers of phones to make their phones and/or chargers compatible with their competitors' phones/chargers, and in fact they explicitly recommend against that exact action.