Add me to those who've never heard of an "eat-in" tax. I wonder if it's a regional thing.
If you're doing it to avoid a tax, it's dishonest. If you're getting your food in a bag because it's easier to carry your food in a bag (and take home leftovers), it's fine.
I am always asked if I am eating in or taking it to go, and I'll say eating in, but please put it in a bag so I can get it to the table - I usually say this while trying to juggle a toddler and my wallet at the same time. There's no way I can get a toddler, stroller, high chair, and tray of food to a table by my lonesome.
I think it's hard to judge someone's motives and know for sure if they paid the eat-in tax or not unless you were directly behind them when they said "to-go".