Not sure if that’s the real reason, as many other test equipment and DMM makers, including all the A brands, allows user update if there’s provision for it.
If I can routinely download firmware for my car that I drive the family around, liability is not a good excuse. In fact, a vendor should makes its firmware user updatable in case bugs that are safety related ever surfaces, so they don’t have to take back out of warranty meters for update.
I think it is more of a corporate stubbornness, as well as so few handheld competitors offer firmware update, that drove the policy.
I have an early unit with old firmware, it does bother me that it is not upgradable. I like the meter, but Bryman could have made it so much more appealing if they allow the user to update the firmware.