As @zenvelo and @cazzie say, it depends on the country and the situation. Each country has their own rules about what makes someone a citizen. For some, you must have been born on that country’s soil (regardless of which country your parents are citizens of) and for some it depends on your parents’ citizenship status.
I found this site that lists a lot of countries and their citizenship laws. It even addresses if they recognize dual citizenship. I hope it helps. :)
According to that site, the Phillipines do not grant citizenship automatically by birth on their soil. They do grant it when at least one of the child’s parents have Phillipine citizenship, regardless of where they were born. So, for Tebow, I’d guess that most likely, he is not a citizen of the Phillipines, but he is one of the U.S. since his parents are U.S. citizens (as long as they had lived in the U.S. at some point before he was born).
As far as if it’s fair or not. I’m not really sure. I think there are so many issues with citizenship and immigration kind of ties into those, so it’s hard for me to say exactly where the line needs to be drawn. Each country definitely needs to make it’s own lines and base them on what’s best for their country though.