Because old people with arthritis can't use them, so hungry lawyers suing on behalf of the ADA will bankrupt you.
I'm fascinated by this statement. Where in the the world did you get the idea that people can sue you for making a product that isn't ADA compliant? I'll admit I haven't read all five titles in their entirety, but I'm pretty sure there isn't anything saying that consumer products must be ADA compliant.
stonecoldsoldier, the real trick for you is to get on the list mentioned here:
http://ag.ca.gov/firearms/fsdcertlist.php
If you can make a device they will approve, you will be in good shape. If they don't bless it, it doesn't count. Where I think you'll likely run into issues is the definition found in section 12087.6 which says
"Firearms safety device" means a device other than a gun safe that locks and is designed to prevent children and unauthorized users from firing a firearm. (Emphasis mine). While you might be able to use hand strength for young children, that certainly wouldn't apply to "unauthorized users."
Here's more:
12088.2. (a) No later than January 1, 2000, the Attorney General shall commence development of regulations to implement a minimum safety standard for firearms safety devices and gun safes to significantly reduce the risk of firearms-related injuries to children 17 years of age and younger. The final standard shall do all of the following: (1) Address the risk of injury from unintentional gunshot wounds. (2) Address the risk of injury from self-inflicted gunshot wounds by unauthorized users. (3) Include provisions to ensure that all firearms safety devices and gun safes are reusable and of adequate quality and construction to prevent children and unauthorized users from firing the firearm and to ensure that these devices cannot be readily removed from the firearm or that the firearm cannot be readily removed from the gun safe except by an authorized user utilizing the key, combination, or other method of access intended by the manufacturer of the device.
(Emphasis mine, again)
As you can see, the standard mentions children 17 and younger. 17-year-old "kids" have about the same hand strength as adults, so that would be a problem. Also, the part about key, combination, or other method... I'm not sure you could work that part out. Would "great pinching strength" qualify as another method to stop a 17-year-old child or other unauthorized person? I think that would be a hard sell.
Hope that helps.