Why do we laugh at someones else's expense
Because they thought they were all getting free money. They gave no thought as to where this money was coming from. Did they think it was magically appearing out of nowhere? They know that, as a betting exchange, Betfair simply brings punters and layers together. The killing they were 'guaranteed' to make was going to come out of the pockets of 'mug punters' - people just like them who just happened to be 'not as clever' as they were.
'Oh aren't we smart?', they all thought. 'We're all going to be rich - I can't believe it's this easy'.
The reason why we laugh at them is because they have all got their comeuppance - they have been pierced by the very sword they were wielding.
If I saw on some forum - 'Hey, here's a guaranteed way to make tons of free money' I would suspect there was a catch, and would do my own analysis to figure it out. If, and only if, I could work out exactly where the free money was coming from, would I take part.
As an analogy, from time to time we may be alerted to big value poker tournaments (like the current Mansion $100K). With the big overlay in this, there was something like $50,000 of free money every night. Now, it was clear where this money was coming from - Mansion were funding the tournament as a loss leader in order to get signups and people depositing and using the software. We know Mansion are a hugely money-rich company, so there was no danger of them not paying out on the value. It was a safe 'free money' as we could see exactly where it was coming from, and see the full thinking behind the money being there.
These idiots didn't do that - they saw 'free money' then piled in without fully knowing all the variables in their market. Betting exchanges are
purely people willing to risk their own money on the basis that they have better information about the market than others do - information which is good enough to cover an commission and make a profit. If your knowledge of the market isn't as good as you think, you'll lose money. If you were so cocksure of profits that you put all your money on it, well, what's that saying about a fool and his money...?