I have clicked on this question about 10 times now as each time it appears in the latest posts box I think it's a clue I may be able to help with, so (with apologies to Baldric) I might as well comment now rather than make it another wasted visit.
I think the papers know they will lose quite a lot of customers from this but they feel it's probably worth it in the long run. I do the crosswords for fun so rarely post them off and would certainly text an answer. The chances of winning are so small it's not worth a stamp or a text, but I can see some people like to live in hope. The post is a very inefficient system for crossword competitions and leads to hundreds of thousands of envelopes being stored then shredded each week. I doubt any petitions, letters of complaint or posts on here will lead them to change their mind. The only way they may reconsider is if sales drop significantly.