That’s a question that you ought to answer yourself, by clapping along with the song. Maybe you’ll find the hidden meaning there, and get over your issue with those who choose to for themselves. I’m sure if you posed to question to each individual in turn, you would get a different answer.
I recall recently making a point with a friend of mine. He took me out to the concert tour How The Grouch Stole Christmas when it came to San Luis Obispo, CA, and at the very beginning and all through the night, the performers encouraged everyone to wave their hands to the beat (it was a rap concert). My friend had just broken his arm at work, and in the spirit of the night, I kept my arms in the air for much (nearly all) of the 3-hour concert. My arms were very tired at the end, and I felt like a goof the whole time, but it was great nonetheless!
I too sometimes have problems with others singing along, clapping, or doing set dances (like in Church gatherings), but I often find that the more people get into it the more fun we all have. The latter is also a cool way to remember the songs, and children in particular often need that stimulus to keep their attention. One way to think of it is keeping that tradition going for the next generation, so that our traditions and histories aren’t lost in ignorance.
That same friend recently has researched that hearts often and almost always beat at the same tempo as the music they’re listening to (ie. people who listen to fast rave music often have premature heart attacks. They also act hyper, getting an adrenaline rush from the process. People who listen to classical slow music have a milder temperament and think through things before acting. That’s why parents are encouraged to let their children listen to Mozart). I would guess that the reason people clap has much to do with this fact; they are keeping the rhythm to the song and their own hearts. The activity also helps the blood move in that same beat, because of the physical exertion and pulse they are actively creating.
A lot of musicians and bands encourage clapping because it does get the audience involved. It’s fun and entertaining for the audience to feel a part of the show. When people are recording in the back, they aren’t just picking up the band’s music. It’s a great thing to know that a recording you see or hear after the fact has the tiny sound of your own clapping helping the beat somewhere within it.
Don’t you ever listen to a song you like and sing along, or hum it later? It’s a similar concept. Ever see Stomp? They actually encourage the audience to participate by clapping, and getting them active. Teaching the audience (like a bunch of children) to make music with their hands; an experience they can carry on to their own children later on. If you had a song, wouldn’t you like it if others wanted to clap to it or sing along? If you were playing a cover song, would you begrudge your audience the right to sing it as well?
I had the unique experience (again, including that friend) of seeing Worldwind at a party here in SLO and being so moved that I sung out (not with words, but a general happy sound, long and melodious as I’m often known to do), while the band was playing. I had a great time dancing about with the people there, feeling the music in me and letting my body absorb the beat. Later that night, when we had lit a fire behind the barn where it all took place, the lead player who had seen me singing, as well as I had seen him playing, took a spot to warm himself by the fire right across from me. I could tell he was about to say something to me (I know not an encouragement or reprimand, or perhaps a job offer?) but was interrupted in his thoughts by a guy next to me complimenting the singing he’d heard halfway through the act. He thought it was genuinely cool and wondered at who had sung, because all the players were so into their instruments. The guy just smiled and said thank you, not mentioning me at all. I knew though, that whatever problem he might have had at the time was completely past and forgiven.