Well, I completely understand your point. I also have rechargable AA batteries laying around, but I see it from this standpoint:
- The Steam controller includes this magnetic charging puck, which is a good way to make sure it's always charged.
- It will mostly be used more or less stationary, sitting down in front of a PC. So even if the need arises to charge it this shouldn't be a problem in many situations. This is vastly different than with the new VR controllers, because they will be moved around a lot and it's not really realistic to charge them while using them.
- Many people still just use disposable batteries. Which is quite frankly just not good. So my best guess is that this connected with point two may be a leading factor for why they did it this way. I think a hybrid option (puck-rechargable battery pack or two AA's) would have been awesome though.
- Valve's repair policy was exceptional with the steam deck so I have reasons to believe that the battery for the steam controller will be available for a very reasonable cost. A very interesting question will be for how long.
All in all I understand your point, but it's not a huge issue for me personally.