This is covered by the principle of the diffusion of responsibility. One person may have morals but spread over many, the entire structure does not. Those corporations that do have morals are outcompeted by those that don't, in terms of amount of money made, so over time corporations that are not moral do better.
Using nginx might enhance a bit performance, but compared to other modules of the overall environment such performance gains are neglectable and unnoticeable for a normal user.
In our latest product I wanted to switch to nginx but I was surprised that for windows it is only in BETA version, and since our application MUST be deployable to both Windows and Linux environments we preferred a consistent software stack hence we stick on Apache.
The plan is to reconsider nginx once it officially supports windows, since no serious customer would accept a beta version for a production environment.