I worked in public accounting in tax.
If they're being taxed for income THEY ARE NOT ACTUALLY RECEIVING, that's incorrect and they are doing their taxes incorrectly.
In the United States, servers are not taxed on income that they do not actually receive.
In the United States, servers are not taxed on income they "should" have received from a moral standpoint (that they should have received a 15% tip that isn't mandatory). They are only taxed on income they actually receive.
In the United States, for the IRS to assess taxes on income, the burden of proof for receipt of income is on the IRS. So, if you do know someone that was taxed on income, there is proof (be it via bank statements, etc) that the income was received.
I have no idea why you labor under the impression you can be taxed on income you don't actually receive as waitstaff.
*edit: I double-checked this just to be sure, and turns out, the employer is allowed to allocate tips under tip compliance, benchmarked at 8% of monthly sales. Taxes withheld on this assumed tip amount is just that- it's assumed you earned an amount.
HOWEVER, if you have records that prove you earned less (that you received less in tips), you only pay tax on what you ACTUALLY earned.
The US does NOT tax income that you don't actually earn. Tax being withheld from your paycheck over your actual tax burden is returned to you when you file your taxes and get a refund.