Hey Ivo,
Are you using CMC on joomla 2.5 or 3.0 ?

the thing is - a lot of people don't have just 2 fields - name and email -> they have interests, birthdate and whatever in their newsletter forms.
So we cannot predict how wide the form is going to be. that is why there is only vertical form and people that want a horizontal should style it themselves with css. Yes, I said - css!

To be able to help you - I'll need to your website and to the published form. Then I can tell you what to add to your css file so that it will look good.

If you already know how to deal with css (which is pretty easy actually), inspect the page with firebug (for firefox) or inspector (for chrome) find out which fields need to be modified and add it to your template css file.

Regards,
Daniel