Siftery kind of annoyingly added my OSS project to it and started emailing me about it without me signing up for it... (months ago)
Apologies Matt - we'd love it if you gave the platform a try, but will not include you in any future email-campaign, going forward.
In essence, the law prohibits business practices that are an "unacceptable nuisance". While advertising via email spam is clearly called out as such, not all cases are so clear cut and will depend on the judge's opinion of acceptable behavior.
http://www.gesetze-im-internet.de/uwg_2004/__7.html
EDIT:
English translation of the law: http://www.gesetze-im-internet.de/englisch_uwg/englisch_uwg....