Many restaurants have take out available, but it's a "table waitress" who needs to stop what she is doing and handle those walk-in and take-out customers.
Many waitresses will pretend they don't see a take out customer, as they rush off to wait on their tables, especially when the restaurant is very busy.
I tip on take-out as a result, if it means that I get service. The waitress says, "Can I help you", and then has to go find my order, take my money, give my change, etc. It probably takes 5 minutes, which is a long time if orders are piling up.
I can see tipping less than a table, as they order drinks, then order food, etc, and it requires many trips back and forth but it's broken up into manageable units and you can often wait on 2-3 tables at a time. But a take-out order simply interrupts that work flow, with no reward at all for the server.
So yes, I tip on take out unless the service is atrocious or they have a cashier who does those orders stationed right there.