Are you the only one claiming 100%? If so, then my thought is that the restaurant is doing this to keep from being audited. If you're claiming $500 each week in tips and all the other servers are claiming $175, it increases the likelihood that the IRS will red flag the restaurant. They not only would audit all the servers, but the restaurant itself would likely be audited as well. Not to mention the reports the restaurant will have to generate...Bottom line, it's a huge aggravation.
That being said, they have no legal grounds to lower your reportable income. Any more than they have legal ground to raise the reportable income of someone they don't believe are claiming enough. Before anything, I would speak to the managers and talk to them about what you think they are doing.
Other than talking to them, I can't think of a scenario that would allow you to keep your job. You can contact the IRS, but I sincerely doubt your employer will allow you to stick around.