^^^^
Well, by saying Encrypted BL, I actually meant, BL locked with encryption (and NOT the certificate check which avoids flashing old firmware even in rooted phones with unlocked BL)... Sorry, if I confused you..
All
Android phones comes with Locked BL, incl Nexus.. But Nexus phone's BL can be unlocked easily (fastboot oem unlock) with simple one line command..
Some OLD phones came with UnSafe boot-image, eg: Old
Samsung phones till S2/Note2 afaik... They can also be simply rooted, by first flashing custom recovery and then making changes in system there..
But OLD
Motorola phones and all new mostly subsidized (market leader/monopoly like Verizon, AT&T) carrier locked phones comes with Encrypted BL. It can only be unlocked via key, but its available for FULL RETAIL (also called Developer edition phones) paid phones only and NOT subsidized ones...
Even in Android KitKat, rooting has become very difficult or almost impossible (esp in ver 4.4.4) to root without unlocking the BL.. For the phones with encrypted BL and running 4.4.4, getting the BL unlocked (with appropriate key or hack) is the best solution... I have paid 50+$ to buy the UnLock code for my DroidMAXX (prev version of DroidTurbo from Verizon, USA) from one chinese middleman and then only I got root access.. (Even the Sunshine hack by JCase won't work for 4.4.4 Moto phones)
Android has become very secured (by patching security holes in each and every version) that UnLocking the BL is the only left out (and straight forward) method to root the phone.. Earlier methods like zroot, faramroot, towelroot (these hacks actually use security holes to get root access) may NOT work in Lollipop, let alone the good old one-click methods...