An ETF is another way to pay the amount you promised to pay when you signed your Service Commitment.  It’s not a punishment. Once you complete your term or Service Commitment, you can cancel the service without an ETF charge.
The ETF amount depends on the type of device you bought. It also depends on how much of your term or Service Commitment you completed when you cancel service. The ETF amount becomes less each month of your contract. When your contract ends, the ETF amount is zero.