Answer: The Monrovia online catalog shows several of these shrubs grown as container specimens in topiary form, so I am not sure where you are reading it should not be grown as a container plant. The problem in your case however is that you are trying to maintain them as container plants in a cold winter climate -- which means they must be overwintered indoors. This is going to be stressful on the plants.
Eugenia myrtifolia 'Monterey Bay' (Syzygium australe) is not very cold tolerant, so you would need to bring it indoors before the weather turned too cold to avoid cold damage and also to avoid shocking it due to the temperature change. As a rule of thumb, I would say bring it inside before temperatures drop below 50 degrees. It should be kept in a draft-free location at between 50 and 65 degrees indoors in a place with high humidity and very bright light. Watering would be a bit less during the the winter (the soil should never be saturated or sopping wet, just slightly damp) as it will not be growing much if at all, but the soil should not be allowed to dry out, either.
Foliage loss can be caused by many different factors including shock, cold damage, over or under watering, dry indoor air, overfertilizing, and even pest or disease problems. It is difficult to diagnose long distance, so I would suggest you work with your local county extension and the professionally trained staff at the retailer where you purchased the plants to see if they can be saved. As far as any possible warranty, that would be based on the policy of the retailer where you purchased the plants. I'm sorry you are having trouble with your shrubs.
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