Have you run across many feral cats? Cats in the wild suffer. I think I mentioned this already, but they are usually covered in parasites, don't have medical attention, often are nearly starving, can't handle cold temperatures very well, and regularly get badly hurt in fights with other animals. I don't see why you think that putting a domestic animal into that situation is not cruel. Could you elaborate?
If we want to talk about effectiveness, I'm afraid you've been misinformed. Euthanizing is more cost effective at controlling feral cat populations, and works far better than TNR (http://www.ncbi.nlm.nih.gov/pubmed/23009077 and http://www.ecologyandsociety.org/vol15/iss4/art10/ and http://www.researchgate.net/publication/242278328_Evaluation_of_euthanasia_and_trap-neuter-return_%28TNR%29_programs_in_managing_free-roaming_cat_populations). If run at very high percentage rates, TNR programs can work . . . but in the real world that doesn't happen - TNR programs across the US average less than 1/7th the needed rate to reduce cat populations (http://www.sciencedirect.com/science/article/pii/S1090023314001841).
I know that killing a pet type animal is unpalatable, but in this case it's the only morally responsible thing to do. If fewer pet owners were irresponsible this wouldn't be an issue . . . but if we can't make people take responsibility for their actions, we at least have a moral duty to try to limit the damage they're doing to the environment. Even if it's hard.
From a biological perspective, GuitarStv is totally correct. Most ferals live short and brutal lives. Just the other morning, we were enjoying a quiet cup of coffee in the backyard and heard hysterical barking and meowing from a couple doors down. Fenced back yard, house owner was out. Her yip yip poodles were tearing a tiny kitten apart. Eventually, my husband hand-climbed over her fence to get the kitten away. It was shocky, and we knew the local shelter was full. We knew we had rescued it so that we could give it a quick death rather than a miserable slow one of torture. Fun morning!
It is depressing, but the most practical, least painful, and most effective control is just shooting them, or trapping (with close trap attendance to minimize the cats' stress) followed by drug euthanasia. We have done that many times in our backyard when the neighborhood population gets overwhelming (one summer, I counted 18 different ferals coming and going in our yard, along with about a dozen pet-cats from the neighborhood). That is some insane assembled predatory intensity.
One thing that angers me is that there a lot of nasty people in the world who actively hate cats, and can be really cruel to them. The biologist community is not exempt...some of whom use the feral cat problem as an 'excuse' to be malicious.
Anyway, I have owned multiple cats my whole life, and we have made big efforts to take in and foster out young feral cats over the years. But when the numbers get too large (which is pretty much any number over a couple per year in our neighborhood because our no-kill shelter is always overrun), we kill them. We don't enjoy it, and it really ruins our day. But the wildlife species are in far greater trouble population-wise than cats will EVER be (for reasons several people mentioned).
There are a lot of emotions around this issue, and I understand that. From a biologist perspective, we have similar emotional conflicts over issues about mustangs (non native, problematic, most practical thing would be to eradicate them), situations where deer overpopulate and starve because (Don't Kill Bambi!), etc. It just sucks.