Sharing Course Material With Other Lecturers

Whether to share-or-not (assuming that you yourself, not your university, own the rights to what you've created) is strongly a matter of personal preference... I don't see a universal mandate.

Many years ago, I did pay careful attention to copyright control, and so on, but eventually I got the impression that it was simply not the case that people were clamoring to get their hands on stuff I'd written. :)

So, for some years now, I've put everything on-line with a Creative Commons license, and whenever anyone is polite enough to ask permission for use, I give it, without any negotiations. In fact, if one of my goals were to maximize the impact I have on ... for example, graduate curriculum in mathematics... unrestricted dissemination seems the obvious choice. True, I may not "get all the credit I deserve", but I already have "enough credit" to survive, so that'd not be a tragedy.

A situation where one should be more careful is when one's institution attempts to use-and-discard people who develop course material. In such cases, "getting credit" probably matters much more. Luckily, till now anyway, that kind of thing has not been toooooo much of an issue at my university, although arising sporadically over the years. (I'm not optimistic about the future...)

In summary: if you don't want to share the fruits of your labors, don't. You may change your mind later, and that's fine, too.

I am I just being prudish?

You or your university own the copyright to your course material, protecting your copyright is perfectly reasonable.

Is this common practice that I just never happened to encounter?

It certainly happens; I don't know of any statistics on the frequency, so can't comment on whether it's common practice.

Is there a potential ethical issue with sharing materials that I am missing here?

If your university owns the copyright, then you'd need permission to share.

Assuming you have a legal right to share the materials, as is common in US universities:

• Sharing teaching materials is a good way to improve the quality of instruction. It is the right thing to do to benefit students.
• If you can get teaching materials you have created used widely, this will help you make a case for promotion. Some universities require faculty to be "world leaders" in teaching in order to get promoted to the highest rank. Getting other people to adopt your teaching materials is one of a very few ways to meet that criterion.