You make a very good point.
Eyes in the U.S. are actually watching more and more world-wide soccer games being televised from every continent.
I grew up in New York during an era when NO games were televised until the late 1950's or early 1960's -- just highlight films, one hour, of games from Germany. Then RAI started to show matches from Italy on Sundays one game a week. Later came condensed one hour compilations of games from England followed by one full match per weekend.
Then came the NASL Local NASL stations on cable and a game of the week on ABC Wide World of Sports televised nationally.
Can't forget televised matches of indoor soccer (MISL). I think it was CBS which may have televised to the U.S. the World Cup. Later men and women's World Cup, WUSA, college soccer, even high school soccer.
Soccer on TV now is all over the place, and not just on weekends. Sometimes on cable youI can flip at the same time through 8 or 9 different games. Sometimes the same game being televised on two stations, one in Spanish, the other English.
So, you are correct. The Gallup Poll doesn't mean the numbers are MLS watchers, but no doubt, there are more viewers than several years ago watching MLS. And that can be verified by the increasing number of sponsorships which MLS signs up to advertise.
Next generation will be watching televised pro video game?? May be a good thing I won't be around by then any more.