If the city council maintains spending at the same level as this year's, it can be expected to levy a sales tax of 2 ...

How come, in the passage (and I'm assuming in other passages on the LSAT), we can manipulate the variables (L2%NY & MSSL) by negating them to create an argument structure, but we can't do the same in Answer B? In other words, why cant the variable "reduce shoplifting" be the same as NOT "suffer reduced profits"? How are we able to decipher a phrase as the logical opposite of a variable?