It is because they both use the same underlying control that has the same name for the validationgroup. I can fix this for the next release of mojoPortal to make the validation group unique for each instance.
However, I cannot really see a good reason to have multiple instances of this feature on a page since one instance can support multiple newsletters. I don't think having 2 of them on the page is a good idea for user experience. It was certainly never expected when developing this feature that it would be used with multiple instances on a page.
If you have 2 newsletters and the user wants to subscribe to both of them he should not have to do it separately or enter his email address twice, one instance can show checkboxes for both newsletters so he can subscribe to both at once.