Your description doesn't match the image you linked

If you are trying to post this decades old image, single images are usually best placed in the appropriate forum.

could you please tell me why are they called world champions(not specifically the celtics, but all who win the nba)? as far as i know the nba is a league of one nation.

The NBA, although in the US, is comprised of the best players in the World.

Can you tell me what team or what league anywhere in the world that could beat the Celtics right now?

Same reason some young lady from Earth wins the miss universe contest every year

I don't see any logic in this statement.
There is also the FIBA World Championship, I think the winning team there should be called world champion.

now-now, if the manchester united wins the premier league they are not considered as the best soccer club in the world just because they are the best in their own country. nope, the celtics arent world champions they are the best u.s. basketball team. and please note, i'm not against celtics or against nba or something. i just think that calling them best in the world after winning a national league is inappropriate. dispite this, they could be the best in the world, but for that they should win a tournament where teams play from all around the world.

Come on, man. You know why. It's the US, we aren't even aware that there are countries outside of our borders. The World Series should get a name change too.

Better yet, let's just abolish baseball and start over again.