Hello xmusic - That disclaimer is included in some songsheets merely to point out that the copyright to the song is owned by somebody other than the maker of the tab. Learning the song and playing it is no problem.
However, if you were to try and make 'big bucks' off the song that would be considered 'improper use'. For instance, if you were to cover a song and then sell a recording, the copyright owner would be owed some of the money generated.
Some internet tab sites are currently battling it out with the Music Publisher's Association. Since the sites make money selling advertising targeted at the users of the tabs, the music publishers feel they should recieve compensation from the tab site owners.
Copyright laws, 'fair use' and other topics are complex legal issues. The copyright police will not swoop down on you and put you in shackles for learning and performing a song (unless you are making big big bucks and are truly worth going after).
"That darn Pythagorean Comma thing keeps messing me up!"
[url]http://en.wikipedia.org/wiki/Pythagorean_comma[/url]