If Canon were protecting their 'monopoly' wouldn't they change the tech slightly each year to make it harder to copy?
That's exactly what they do as newer cameras are able to tell original Canon and Canon copies apart while older cameras keep recognizing all as valid.
If Canon were gouging it's customers, wouldn't it make the batteries different so every time you upgrade, they get more cash?
No, because you can only milk your customers for so much, and you have to have at least a somehow recognizable reason for doing so.
Take mobile phone manufacturers: They kept changing their power adapters so much that the EU intervened and make them implement a standard so prevent electronics garbage. Then they all had an own way of sticking a headphone to a mobile phone until customers were fed up about it and started buying phones that have a standard jack.
Canon might be a big manufacturer, but they don't have a monopoly so customers do have a choice ... and accessory continuity like old battery models in new models is an incentive to stick with the brand. Last not least, changing hardware production like batteries for each model is expensive for Canon, that's why they keep recycling parts like the af arrays and so on.