Have you changed your notification preferences under your profile?I've been noticing those too. The last batch I got however, at least were correctly labeled as being from Gretsch-Talk and not Drum Forum. So ... that's progress, I guess?!
They also weren't technically spam in that they weren't advertising vacumm cleaners or something. Instead, they were a summary of the site's latest posts, more a newsletter than spam.
Personally, I think they're a waste of time. If I want the latest posts, I visit the site! Why would I need an email about it?
But I get similar emails from several other sites. Seems to be the latest fad in marketing right now. Some idiot's idea of a good idea!