The belief that watching TV is wasteful comes from the experience of actually wasting time watching TV when one would prefer to have done something else. Why don't we do something else? That's a good question, but not the question you asked.

A better example of social conditioning is "common sense". If you ask a question and the answer is "common sense", the likelihood is that the person is simply socially conditioned to think that way. An example that I've given a talk on is that Japanese people sit down while showering. Western people stand up while showering. Explain why one is better than the other. I did this in a workshop composed of half Japanese people and half westerners. After 10 minutes I had to halt the exercise because people were close to coming to blows. It was amazing. The answer, of course, is that it doesn't matter really (apart from preference for a couple of minor details), but I was not prepared for the backlash of unthinking cultural beliefs.

Perhaps a better question to ask would be "Is it better to read a book or watch TV"? There I would agree with you that people are socially conditioned to respond with "read a book". They don't actually know why a book is better and, in fact, are unlikely to have ever thought about it in detail (and almost certainly have never looked at any studies on the matter). They just know it to be true (for various values of the word "true").

However, I think these socially conditioned preferences are not in the majority. People's preferences are actual preferences. Someone likes meat, another likes fish. You can learn to like something else and you may be pre-disposed to like something based on familiarity, but that's not social conditioning. People do think about their preferences quite a bit, even if they are influenced by others. People like pop music, not only because it is popular, but also because the genuinely like it. If you ask them what they like about a song, they can actually tell you in surprising detail. Often they like some popular songs and don't like other popular songs. That most people enjoy the same songs in a culture is not surprising -- that's familiarity working. You like what you know.

One of the reasons I responded to this message is because I think it is a big mistake to underestimate people and classify them as a kind of "sheeple". They aren't a big unthinking herd, just following the person in front of them. There is an aspect of that, yes, but it is really dangerous to imagine that this is all there is.

It's easy to convince yourself of the opposite position due to confirmation bias (there are lots of examples of social conditioning), but I recommend looking for the opposite. I think it will surprise you.