Good question! On the surface, it may look like these cows are happily giving away their extra milk, until you consider the factors of the situation:

• Cows, like other mammals, produce milk to feed their offspring. Dairy cows need to be regularly impregnated, generally through artificial insemination, in order to keep up milk production.
• Since the newborn calves would otherwise drink their own mother's milk (leaving less for the dairy industry), industry practice is to separate them as soon as possible. Dairy cows have been observed mourning their lost calves.
• Dairy cows have been selectively bred to produce much more milk than their closest natural ancestors. This results in higher rates of mastitis, as well as pain and discomfort from the sheer volume of milk production.

These cows are "freely" choosing to be milked because they've been bred to produce painful amounts, and they no longer have calves to drink it.