Quick answer:  they can be if they are misused.  A properly fitted, time limited, soft muzzle if not cruel, what would be cruel is to let your horse get too fat and get laminitis. Laminitis is so painful to the horse that it is cruel to them, you can affect this and using a muzzle will help keep them safe.

I hate muzzles, there is no other way to say it even if I did invent the LiteBite Horse Muzzle. The reason for inventing it, which took years to get right, was that after laminitis due to being too fat and I didn't know the risks then.  I needed to stop her eating far too much, she seems to put on weight from fresh air (me too I'm afraid)  so I bought almost every muzzle on the market, spent a whole lot of money but she would not eat with any of them. How miserable can a horse look?  Very.

The Litebite is different, apart from being soft and very strong, it has a unique grazing slot which allows a horse to just graze as normal but the narrow slot restricts the actual amount of grass eaten.  She grazed happily in less than half an hour.  What a relief.

If you have this same scenario going on, just try the Litebite, I think you will be pleased. Full refund if not of course.