I`d have to agree with requesting permission to link to another site`s articles, resources, etc. from your newsletter.
Occasionally, we receive requests here at StartupNation from someone that would like to feature one of our artcles or On Demand seminars on their site or in their newsletter. We have never declined and are glad to know that our resources are of value. However, we do have certain branding and copy guidelines that we ask them to follow.
Are we going to sue someone for not asking us? No, probably not. (unless they`re used in an unethical manner)...but it`s still nice to be asked.
(The site you link to may also track how many people clicked over from your newsletter..and if they are impressed...you may have yourself a valuable new business ally.)
Putting links to other sites in your blog or simple resource links on your site shouldn`t require permission. I`d just use your best judgement with these.