It largely depends on the context the "you should give this away" is said in.
If say a well known media organization says, "You should let me use this photo for free, because I will give you credit and that might help you get business later" that is negotiating. The creator of the photo may or may not like the terms, but it is an offer of something of value for something of value. At worst, if the photographer is already well known, it is a bad offer that is wasting his time, but it is never insulting.
If someone who studies economics says to a new photographer, "You should stop selling photos and sell the service of creating new photos on order due to the marginal prices involved and current market situation. Give your old ones away to build up reputation." This is not an insult, but advice (whether or not it is good advice is a different question).
He is entitled to set the asking price for his work, but that is all.