Welcome to FT csauvain!

I don't know what an agent told you. I find they sometimes just don't understand what you are asking. You can find information regarding traveling with infants here, on UA's webpage:

http://www.united.com/web/en-US/cont...s/default.aspx

Infants traveling without a seat to other international destinations are charged 10% of the adult fare at the time of infant ticketing (it is usually less expensive to purchase the infant ticket in advance). Infants traveling on an adult’s lap on front cabin rewards or upgrades must pay 10% of the front cabin fare in applicable markets.


It's just the way it is.