Some ISPs reject posts if their domain is not part of the email address. Some require that posts are made from one of their IPs. Some require both.
Newsgroup providers can see the size, subject and attachment names of all you post through them. Whether they choose to keep the server logs that are always created and details of your usage will appear in their policy details.
Given this it would be folly to assume that providers don't know what you're doing.
Providers can see how many groups your subscribed to if any but not which ones can see how much you upload or download other than that they cant see anything else.
Newsgroups is by far the safest way to get anything.
Providers and subscribers alike can read any post's full headers which will include details of the poster's newsgroup service provider and IP.
Providers can see exactly which groups you subscribe to and what you're downloading because server logs detail which IP requested which headers/posts from whatever group. As I noted above, whether they choose to keep these details is dependant on their policy.
Of course, there are thousands of people using any given providers newsgroup services so they're not going to waste time browsing through logs to see who's doing what. If authorities forced them to hand over logs and usage details, however, they will probably comply in order to stay operative. For this reason, usenet is undoubtedly the safest place to download.
On the other hand, posting is not so safe.
As I mentioned above, anyone downloading a post can read the headers and get the poster's IP & provider and the time of the post. With this infomation, authorities can request user details from the provider.
Using providers offering a level of anonymity is obvioiusly important.
All this adds up to one piece of advice:
NEVER post warez to usenet using your ISP.