Great question. Market makers have no particular incentive in keeping the price low. They make money when shares trade hands. When you sell shares, you sell them to a market maker. When you buy shares, you buy them from a market maker. They make money on the difference between the buy and sell price. When volume is high, market makers earn more money. They have an incentive to keep the volume as high as possible, but there aren't very many ways to do that.
One trick they often play is to buy a few shares at a relatively high price at the end of the day. This is the reported share price for the stock. Keeps interest up.
There is no evidence that market makers are working to keep the share price down. Just another crazy theory from people who are looking for reasons why KBLB isn't selling for $8/share right now. After all, why should a company with no sales, no profits, a lot of debt, dwindling resources, a billion shares outstanding, and a CEO who has shown himself untrustworthy and withholds critical information from shareholders not be selling for multiple dollars per share?
Reality can be so cruel. Bring in the conspiracies!
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