This is almost never true with cost-plus contracts. There are normally all sorts of incentives and written into the contract. For example, bonuses might be paid if particular milestones are achieved ahead of schedule, or if certain costs come in below estimates. Also, there are usually limits on reimbursement (typically at the level of components rather than the entire project).
If used properly, these incentives and restrictions make it in the contractor's best interest to control costs. I've never seen them used perfectly, but they're often used reasonably well. I've also seen incentives that were designed so poorly that the contractor just ignored them because they weren't worth the effort to try and earn.
I think the main problem is that cost-plus pricing is often used in circumstances where it is not appropriate. Cost-plus should generally be used only for high-risk projects.