You Break it You Bought it

The user / owner will expect you to warranty your service even though THEY received all the value of your time, and you received nothing in exchange.

People don't respect things that are free

Free advice. Free upgrade. Free entry. None are valued. Free advice is seldom wanted. People associate the value of service with the amount of money that is exchanged for it.

They will expect it forever

When they come back and you try to get fees, they will meet you with resistance in the form of guilt. "I thought we were friends" they cry. "You didn't charge me anything last time." They argue.

The demands will only grow with time

Once you've set the precedent and created the expectation that you are their knight in shining armor, they will begin to call you for everything.