At the present time, echo only recognizes switches and lights. Nothing to do with SmartThings. Same is true for Wink and Insteon.
The workaround is to use a virtual switch to stand-in for the lock. A number of community members, myself included, do that. (I use it only for unlocking from indoors, by the way. But there are other cases. )
You’ll find more details in the community FAQ:
Also, it’s nothing to do with security. Smartthings allows you to use IFTTT to unlock locks (by the way harmony does not) so at that point you have unauthenticated voice to unlock if you want it just using a regular voice text.
And the Amazon security FAQ specifically mentioned locks and garage doors and says just be aware that anybody can use it to open it. They don’t care. They leave it up to the individual user.
http://www.amazon.com/gp/help/customer/display.html?nodeId=201819970
Amazon just hasn’t implemented support for this device class yet, just like they haven’t implemented support for thermostats or Sonos. They probably will eventually. Meanwhile, you can use the virtual switch work around if you want. It’s up to you.