First, what is public ads service in adsense?
PAS (Pubic Ads Service) is the ads that displayed inside your site (adsense publisher) and you will get nothing if someone is clicking your ads.
Why Public ads service come to my site?
Because you are not using the language that google adsense support ; or you are using an forbidden categories article like an adul* and s*x.
How can i solve this problem?
By adding the script below, you will not see any Public ads service again on your site even you are using your national language that unsupported by google adsense.
(image taken from the web)
Is there any penalties if i modified my google adsense script?
YES!! If they were know, or someone reporting them, you will got banned forever ;P