Even if someone thinks it's true that Carnival gives the best prices (which I don't believe is true), they will allow you to book your cruise, then transfer the exact booking to a travel agent, should you find an agent who's willing to book the cruise for less... which again I think wouldn't be difficult to at all.
And, if you happen to be shopping for a cruise, and find a price that's less than Carnival is offering it for on their web site, I really don't think they would match that price.
The vast majority of Carnival's business is booked by travel agents. They aren't about to set out and undercut them, simply because it would harm their business.
Renaissance cruise lines tried that once, and they're no longer in business.