As a house painter and someone who is married to a Hispanic woman, I might be able to give a little insight. While trying to avoid the obvious political hot buttons, my response is this:
1) No you shouldn't feel as though you are "subjugating" them. They are here willingly, working for an agreed upon wage, whatever that may be. You might be surprised at the hourly pay they are actually receiving.
2) If they are second or third generation immigrants, then you are correct, they have had many opportunities afforded them, and the job they are doing is largely their decision. If they are relatively new arrivals, then they are happy (I am not saying satisfied) with their position in life, as it is much better than in Mexico. Also, they tend to enjoy manual labor type jobs - many take pride in quality workmanship. Most of my wife's brothers and cousins are self employed contractors of various trades in Texas. They do this type of work by choice, as all of them are high school grads, and some of them went to college.
3) Keep in mind that they are not doing jobs that Americans won't do, they are doing them for wages most Americans won't accept.
4) You said:
"When I looked at them, they didn't even smile. I myself tried to smile and nod warmly, yet one just gave me an obligatory acknowledgment and went on with his work."This is most likely because they didn't speak English, and they felt inferior. My wife says that many Hispanics (coming from a very friendly and outgoing culture) will avoid contact with "gringos" because they speak poor English. They also may not want to do anything other than exactly what the boss said to do.
Finally, Maybe the only thing you (and most Americans these days) need to feel a little guilty about is that you didn't specify that you wanted all Americans (legal immigrants or citizens - Hispanic or otherwise) working on your house. This would require that you were willing to pay more, probably a lot more for the job. I routinely lose jobs to painters who under bid me by sometimes half, and low and behold, when I see them doing the job, it's a bunch of Mexicans doing the work, with "gringo contractor" nowhere to be found.
It seems to me that we as a society need to decide what our priorities are. We shop at big box stores for the convenience and the price, but complain about the societal affects that they have. We complain about high gas prices, pollution, and dependence on "foreign oil", but our SUV's, pick-up trucks and" beemers" all have V8's getting bad mileage. Similarly we want our home improvements done cheap, quickly, and with decent quality, but then there isn't enough money to pay the laborers a "living wage."