Well, you shouldn’t have to turn UEFI off to boot from the USB stick. That one could actually be on Mint. Or on the way the stick was created. It’s been a while since I messed with this stuff but by now a typical Linux installation medium should be able to boot with UEFI.
The secure boot is enabled by default so that only signed operating systems and not some malware can boot the computer. And I guess it’s legitimate to not have the keys of all the Linux distributions under the sun in there.