You always*or at least I think its you* say this...
You know the answer!
Because nice as it is to get published, by allowing magazines to get away with this you devalue both your own and the photographer's work. If work has any value make people pay. There is a thread going on about the strike on Wednesday, this strike is public sector workers saying that the work they do is valuable and that their pension rights should not be taken away (the European Court in Barber V Coloroll determined that pension was deferred pay).
Magazine publishers make good money, paying £50.00 per image is peanuts, but would at least show that they value the submission. I have no issue with submitting an image for consideration, or entering a competition, but as soon as that image gets printed or put up on a website, money should change hands. If models ever wonder why they cannot live on their income from modelling, this is the reason.