Is it always OK to ask for a copy of the lecturer's slides? Weeks ago I attended a lecture/seminar in NYU Shanghai, and I am not a student nor an alumnus of it. After the lecture I attempted to ask if I can have a copy of the slides because the content is so interesting. The lecturer said to me that I'd better email her later and she will send me it. And I searched Google for her name and found her email address and shot her an email(using my hotmail email address), but now I still have not recieved the slides. 

I thought she doesn't know me and hence might don't know that I am not affiliated to NYU Shanghai. But it seems that she doesn't have any reason to share with me the slides. 

When I was a graduate student I attended a lecture on open source and what the lecturer said about it is still whirling in my head: the slides that are not allowed to share are not valuable enough to be shared. I love such ideas, abeit sometimes too idealistic. 

But when is it appropriate to ask for a copy of the slides and when it is not?

EDIT after one day of waiting and two additional emails:
Following the suggestion by @WolfgangBangerth I sent the lecturer one more email and she redirected me to the host of the seminar and then I cold-emailed the host with some explanations such as who I am, how I relate to the university and why the slides are important to me. Just now I recieved the slides from the host. Thanks.
It is always appropriate to ask. But some people are unwilling to share their slides for a variety of reasons (none of which I think are particularly good, for basically the reasons you mention in your quote).

So you may or may not get the slides, but it is certainly ok to request them!