I think it has more to do with how tax and shit is delt with, taking tips while at "full pay" fucks with how the payroll is supposed to work or something.
The end result is what you say tho, regardless of the reason. In the end you just stay poor.
I'm not saying it's right or anything, just that's basically every stores policy. That's all. I ignored that policy everywhere I worked for example, yes please I will take your tip.