What. How is this even remotely acceptable? If discovered it destroys your credibility. Among your employees it destroys your credibility. If I was an employee and found out that this was happening, I'd be extremely upset. I wouldn't trust a thing you say. If you'll lie to customers, you'll lie to me.
Your optimism is endearing, but the vast majority of companies I've done business with do this. Often companies I've worked for have, too – sales people are very happy to overstate the available features and capabilities of any product. From their perspective, a shoddy, rushed implementation to an unhappy client is still better than having them walk away from the purchase entirely.