Exactly. The danger of Ebola spreading is not from a patient in an isolation unit, but from a person who is sick, unaware he or she has Ebola, and interacting with other people.
For example, if someone who is feeling a little sick were to go to a public place and vomit, anyone standing near enough to have bits of vomit splash on them or even breath droplets of vomit would be exposed. Whoever cleans up the mess will be exposed, and people handling the trash where the clean-up materials were thrown away could be exposed. Someone having diarrhea in a public toilet could expose the next few people using the toilet, since the act of flushing the toilet sends droplets into the air as far as 6 feet away. Plus, any residues of diarrhea remaining on the toilet could remain infectious afterwards, maybe for a few hours.
I could list many other scenarios where Ebola could spread. They all would involve an infected person who has not yet presented for medical care. There is also some risk early in the medical care process, when medical personnel do not yet suspect Ebola and so expose themselves.
The two people brought to the US are medical people, who knew they were treating Ebola patients.
What happens when there are 23, or 100 patients? They'll still likely be sent to the centers with the trained staff, and attempts will be made to isolate them, but the degree of protection for the staff, and others in the hospital will be lower. If they are sent to other facilities, with less well trained and equipped staffs, the risk will be higher yet.