I tend to think that most people who think it's just fine don't realize or possibly believe the negative side of it... or maybe just don't care since they are "just" animals.
http://www.inkokomo.com/dolphin/captivity.html
Is it cool to see dolphins at the zoo?
CONSIDER THESE FACTS:
53% of those dolphins who survive the violent capture die within 90 days.
The average life span of a dolphin in the wild is 45 years; yet half of all captured dolphins die within their first two years of captivity. The survivors last an average of only 5 years in captivity.
Every seven years, half of all dolphins in captivity die from capture shock, pneumonia, intestinal disease, ulcers, chlorine poisoning, and other stress-related illnesses. To the captive dolphin industry, these facts are accepted as routine operating expenses.
In many tanks the water is full of chemicals as well as bacteria, causing many health problems in dolphins including blindness.
When a baby dolphin is born in captivity, the news is usually kept secret until the calf shows signs of survival. Although marine mammals do breed in captivity, the birth rate is not nearly as successful as the one in the wild, with high infant mortality rates.
Wild dolphins can swim 40 to 100 miles per day - in pools they go around in circles.
Many marine parks subject their mammals to hunger so they will perform for their food. Jumping through hoops, tailwalking and playing ball are trained behaviors that do not occur in the wild.
Confined animals who abuse themselves (banging their heads against the walls) are creating stimuli which their environment cannot supply. Dolphins in captivity tend to develop stereotypical behaviors (swimming in a repetitive circle pattern, with eyes closed and in silence) because of boredom and confinement. This is equivalent to the swaying and pacing of primates, lions, tigers and bears confined in cages.
Dolphins are predators of fish and spend up to half of their time in the wild hunting for food. Supplying dead fish results in less exercise and lack of mental stimulation, thus causing boredom.
When trapped together, males often become agitated and domineering. This creates pecking orders (unknown in the wild) and unprovoked attacks on each other and the trainers. In the ocean, although fights are not unknown, the wild dolphins have a chance to escape.
More:
http://www.dolphinawareness.org/
http://www.greatdreams.com/eeyore/dolphins.htm
http://www.dolphin-connection.org/information/captive.htm
Captive Born
A high percentage of captive born dolphins are stillbirths or premature.
Captive born dolphins rarely lives to reach puberty.
Dolphins born in captivity are often taken from their mothers. They sometimes kill their own babies. This is maladapted behavior.