According to Google guidelines, paid links such as ads should be 'nofollowed'
Paid links: A site's ranking in Google search results is partly based on analysis of those sites that link to it. In order to prevent paid links from influencing search results and negatively impacting users, we urge webmasters use nofollow on such links. Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships (for example, a full-page newspaper ad may be headed by the word "Advertisement"
So if your link was to be defined with the
rel="nofollow" tag, the answers to your questions will be:
- Does google understand that this is non-natural backlink? YES
- Does google prevent the PR juice to flow in such cases ? YES
- Does it have any negative effect on seo? NO
However if you didn't use the
rel="nofollow" tag, the the answers would be:
- Does Google understand that this is non-natural backlink? NOT immediately, but will more than likely figure it out after seeing it apperas on all these pages
- Does Google prevent the PR juice to flow in such cases ? NOT immediately but again if Google decides this is an unnatural link it will do more harm than good.
- Does it have any negative effect on seo? YES it can do. It could be seen as an unnatural link and you could even get a warning in Google Web Master Tools
More info on links such as this can be found here: Google's Guidelines on Link schemes