It just seems very unethical and a ridiculous practice to do that. I'm starting to wonder what other "fake" stuff is out there.
I do draw a distinction between those who actually claimed solar power vs those who doesn't but instead just use a case that has the capability.
Unless they claim such capability, one should not consider it fake. I had (in the past) a $5000 plus hp managed switch that has empty but covered slot for Gigabit optical (when optical was the only way to get to 1GB). The switch was not advertised as optical-capable. It merely shares the same case (but not model number) of another model.
I have seen many with a real amorphous panel in there, and as well the small button cell, which is actually charged by the solar panel. It is reasonably safe as the panel will only deliver about 100uA into a short circuit in direct sunlight at noon, so the coin cell is very unlikely to be overcharged unless you leave the calculator out in sunlight all day. There is a small germanium diode to prevent the cell discharging back through the panel.
The solar cell does provide a useful extension to cell life, you can see that the display will fade as the cell ages and placing it in direct sun for a half hour brings it back a lot. As they use a common LR43 cell in many cases or a 377 cell replacement is easy, once you undo the dozen or so small screws holding the unit together. I have done quite a few Sharp branded ones that are used at work.
I agree, the power LCD calculator need is so small. Even with the example you shown, a well designed calculator has battery that lasts almost forever. My HP16C from the 1980's are still on its original battery and I use it at least twice a week (bill payment cycles, always double check my mental-math with the calculator). A better battery is probably a better solution.
The energy use in manufacturing, transporting, and installing these baby-panels probably exceeded their energy contribution by many folds... I can't think of a manufacturing process that can moving a 1/2gm panel into the plastic shell,solder it, and test it and that the process would consume less that a milli-Watt-hours (1W equipment ran for 0.3 seconds). Say there is one, it would still be just one part. Forklift loading, tracking, and shipping of them every node...
In truth, I really doubt if they add any meaningful power for the cost to the environment they incur. If
these there is one way to introduce a cleaner environment, this is one of them. There is no need to introduce more toxins into the environment by making these solar cell panels and more cost to the environment in transporting them around.
Rick
(EDIT-typo (strike-out font) corrected