Why do people who don't believe in Cryonics want to slam it?

Even if Cryonics doesn't ever work, then why would people want to stop it. After all, The people who have signed up have a right to spend their own money how they choose. As a society don't we honor these freedoms and certainly we should honor the reasonable last wishes of the dieing. If Cryonics turns out to work...then trying to stop it would be akin to killing people through ignorance...? Even slamming the idea could do harm if it works. Think Africa where Aids patients die because they are told not to trust western medicine. I think people should be careful of what kind of misinformation they spread...or the future blood of loved one and friends could be on their hands...They should research and learn carefully before slam what may turn out to be life saving gift.

People fear death. Most of them also fear the unknown. Shakespear called death "the undiscovered country" as many people us know from Star Trek films. This fear is often expressed in the form of denial whereby, like a bunch of immortal teenagers, even the oldest of us do not dwell long on our own mortality. Most people never do estate planning to set up trusts in order to protect assets and lower the tax debt obligation of our heirs. Most people never set up Health Care directions in the event that tragedy strikes (witness young couples like the Schiavos in the famous Florida right-to-die case from a few years back).
Cryonics represents a future and a state beyond death that flies in the face of what a American Judeo-Christian enculturation has fed them for years.
The unknown is like the stranger in the village in the olden days. Get enough drunks hanging out at the pub, they'll eventually find a way to blame the new guy for the economy, a missing person and a niece who's pregnant, and suddenly the poor sap is being run out of town at best or lynched at worst.
Indeed, as you've assessed correctly, cryonics is the best potential that people have of seening the next millenia, barring the development of excellent anti-aging technologies. Every other death-related option has little liklihood of revival barring only an extreme nano-revival coupled perhaps with virtual simulation extrapolations of documented people and cloning of the physiological form. Cremation is surely the worst of all.
Stop it? No. Regulate it just as the funeral industry, morticians, and other dealers in death are held to certain minimal statutory standards seem reasonable. This would lend it two things: expensive permits with oversight, and the stamp of legitimacy from government approval.
Worst case scenario with cryonics is that you're still dead and never revived and that you've directed one single life insurance policy to a nonprofit corporation who used those funds to advance the causes of medical and scientific research. New blood substitutes (Cryolife, Inc.), new techniques for whole organ preservation for transplants (BioPreservation, Inc), early experimental testing of super-clot busters like Activase (Alcor) have all been pioneered by cryonics firms. These are clear benefits to society. Cryonics contributes greatly, and not just to its' frozen wards.