First I got the code by visiting https://www.addthis.com/get/sharing
Then I inserted it into my WordPress site: dashboard>Settings>AddThisShare>Basic>Custom Buttons
I found that the same problem was mentioned here on October 2012:
addthis wordpress plugin strips script and analytics in custom button code box
The Paul answer to contact the software vendor seems out of topic.
The software is Word Press 3.6 and I assume that even previous version had the same problem (the question was posted on October 2012)
My AddThis Social Bookmarking Widget version is 3.1
Either version won't affect the issue. IMHO the problem is related to the visual editor used by the widget for inserting the code.
Any workaround will be appreciated. I just need to use only certain social networks and not the standard ones proposed by the widget.