I really do not think it is in anyway cruel. This action is to prevent squirrels from eating the birds food. I put netting over my plant pots to stop the squirrels digging up my bulbs it is not harmful to them.
The squirrels are able to easily get a more varied diet than a lot of birds who seem to have a more limited range of things they can eat. Squirrels prefer the easier task of helping themselves to the "fast food" we leave out for the birds.
www.ypte.org.uk/factsheets/hibernation
"Squirrels' diets consist primarily of a wide variety of plants, including nuts, seeds, conifer cones, fruits, fungi, and green vegetation. However, some squirrels also consume meat, especially when faced with hunger. Squirrels have been known to eat insects, eggs, small birds, young snakes, and smaller rodents."