Employers are required to withhold income, fica and medicare taxes on reported tip income. Certain places have allocated tip income because of agreements with the IRS as well. While I agree with LdiJ a flat 20% is probably not correct, without the numbers in front of me it is hard to tell.
When you get your W-2 this year, be sure the withholdings are appropriately reported there. If so, you really don't have a problem you can deal with. (Except that you can argue with your employer he is doing it wrong if the percentages are not right.) If not, THEN you have a cause of action and can think (in the back of your mind) you are being cheated.