As a general rule companies will go up and down in value, and there's no reason that the same company cannot be sold and bought back at a later date. To require the seller to retain an equity stake would be to go against the basic principles of selling - if you are retaining an equity stake, you are not really selling at all. Additionally in the example above one might argue that AOL did not adequately manage Bebo so perhaps the loss is entirely of their own doing.
In business, people buy and sell regularly. The business entities are free to do so as long as they can afford to. Requiring that interests be aligned is not necessary. While some laws need to be in place to protect consumers and the environment, I don't feel that forcing sellers to retain an equity stake is a good idea.
This case is something of an outlier. It tends to be the case when companies are subsumed into a large group that they become more profitable. It is very unlikely that large companies would want smaller partners holding on to an equity share in all cases after a buy out as this would also make them eligible for a cut of the profits.
To require sellers to retain an equity stake to align interests should not be implemented. If a company is sold one year for $20 million and 15 years later the company is worth $5 million that is simply the state of the market and the companies value at that time. There is an amount of risk involved in investing and purchasing of companies, the buyer takes on that risk as long as they own it.