ASICminer did decide to keep the hardware and mine. They're selling shares with intent to payout the dividends from whatever profit is made.
However, Avalon and BFL didn't choose the same path. As to why, some people believe it's far better to "sell the shovel" than "dig for gold". No one knows what the exchange rate or mining difficulty will be in 3 months (things that affect profits for miners, along with electricity), but none of that matters to the hardware companies, since their profit will already be in the bank.
And the increased difficulty is precisely why the machines won't pay for themselves in 5 days. The might if you had one right now, when difficulty is low. But the network hash rate is going to increase by a large factor once all the ASICs have shipped by this summer.
Eventually, your single 50GH/s machine will be unprofitable as new, more efficient technology debuts and the network difficulty continues to climb.