Most recent phones lock the bootloader by default, regardless of whether or not decryption is enabled. Some phones, such as the Nexus series, allow it to be unlocked relatively easily. Others, such as those made by Motorola, require the user to go online and get a unique unlock code for their device. And some, such as LG, don't allow the bootloader to be unlocked at all.
In practical terms, the only people who would ever unlock their bootloaders are those who wish to perform modifications (ie. rooting and customs ROMs), and they typically accept a somewhat lessened amount of security anyway.