I don't think it's "unfair" or cruel to keep a horse stabled but I don't consider it to be ideal either. Personally I prefer horses to have a more natural lifestyle if possible, and my personal definition of "natural lifestyle" includes full-time or almost-full-time turnout. Mouse lives out 24/7/365 and I definitely want to keep it that way, because he's happy and healthy and it works for him - and I have absolutely no doubt that he will continue to be happy, he has now survived a long winter living out in the snow / heavy rain / sub-zero temperatures with no rug so I don't expect he will suddenly start to struggle with living outside :giggle: I don't want him to live in a stable so I will always do my best to ensure that I don't have to!
However, I'm fully aware that living out full-time or almost-full-time doesn't work for some horses (or owners) and sometimes full-time stabling is the more suitable option. If I found myself owning a horse who couldn't or wouldn't live out for whatever reason, I would be fine with keeping him stabled BUT would make sure that he had plenty of physical and mental stimulation to avoid him getting bored / stressed - taking him out for exercising obviously, and grooming / handling / groundwork, and turnout even if just for an hour, and also giving him things to eat and play with in the stable. I don't have a problem with horses living in, providing they receive adequate stimulation (and some horses will need more than others). What I don't agree with is horses living in and not "doing" anything.
Over here it's quite normal for horses on yards to live in, and to have a more "artificial" lifestyle. Riding horses tend to be stabled and not get a great deal of turnout. There are horses who live out, but most of them are youngstock / broodmares / retired horses / horses who are ridden once in a blue moon - around here at least it's unusual to find a leisure horse in regular work who lives out 24/7. I do agree that horses are "made" as outdoor animals, but in some places this simply isn't a part of horsekeeping culture and I fully accept that things are different in different places.
We have 14 boxes on the yard and those horses all live in. We have 7 individual paddocks at the yard, which means we can get them all turned out each day, half in the morning and half in the afternoon. They are only out for around 3 hours though, but this is what they're used to and happy with so it's not like it does them any harm. In addition they are also exercised twice a day (either ridden, lunged, loose-schooled or put on the treadmill) - this means they have time to stretch their legs and things to keep them occupied, it's not like they're in a stable for 24 hours without any physical or mental stimulation.
If there is one day we don't do anything with them - for example if we're having a day out (which is very rare btw!), or when it was snowy/icy and we couldn't ride or turn out because of the ground conditions, or Christmas Day when we gave ourselves a holiday - they are perfectly settled in their stables all day as long as they have hay to nom. Many of them will just lie down and sleep. But the following day they will be showing signs of impatience & when they get their turnout they bomb around like lunatics, have a good roll and generally make the most of it. They are never "abandoned" like this for more than 1 day and I wouldn't find that acceptable.