How do ethics work in Shin Buddhism? "Even the good person attains birth in the Pure Land, how much more so the evil person."

I think I understand the reasoning here-- if practice isn't effective, then the evil person, who doesn't practice or I suppose, do anything good, isn't planning on getting to the Pure Land by his own means, he's going to have to rely on someone else to get him there.

So, after this, if one's goal is to get to the pure land, what is the rationalization for ethical behavior?

(Let me emphasize I'm not trying to imply a suitable reason for being ethical isn't there, I just haven't gone in depth far enough into Shin Buddhism to know what the answer is, and I find it bemusing)
The founder of Shin Buddhism, Shinran, assumed that a person of shinjin (grounded faith) would naturally be inclined to refrain from harmful actions. These quotes are all from various letters that Shinran wrote:

...When people come to have faith in the Buddha deep in their heart, they genuinely renounce this life, they lament their transmigration in Samsara, they have deep faith in Amida's Vow, and they delight in saying Amida Buddha. If these people truly desire not to commit the evil deeds that they may be inclined to do, it is an indication of their renunciation of this world...
...thinking of the Buddha's benevolence, devote himself to the Nembutsu in order to respond with gratitude for that benevolence, and should hope for peace in the world...
...Do not take a liking to poison just because there is an antidote.

It's not really a rationalization, since ethical behavior in Shin Buddhism isn't done to gain something, even though it generally results in a more peaceful, happy life. It is more like a result of the transformative power of shinjin.
Reference: Living in Amida's Universal Vow, &quot;Ethics in American Jodo-Shinshu: Trans-ethical Responsibility&quot;, pg. 201