Well, if you couldn't afford a $50k pickup without taking the interest-free loan, that doesn't apply to what I said about making a choice between paying cash or financing.
In the scenario I mentioned, it's a choice between liquidating investments or financing for 0%. My scenario assumes you have the wealth to buy the vehicle in the first place. You have to have pretty good credit to qualify for those zero interest deals. It's not like poor people can easily qualify, and it's not like those who can qualify are likely to make unwise financial decisions.
The most likely scenario in which this becomes a trap is when some poor person who doesn't know how to handle money comes into a $50,000 windfall, borrows $50,000 at zero interest, and then spends the $50,000 windfall rather than investing it.
Now, if you're talking about the expenses involved in driving, fixing, and maintaining a $50,000 pickup truck, yes, those expenses can come as a shock, but look at what I said again and you'll realize that by its nature, having a choice between liquidating investments to buy a vehicle or qualifying to borrow at 0% interest puts you in a class where you can likely afford to buy a $50,000 pickup truck.
I have yet to meet a person who can qualify for a 0% APR car loan, has the money in the bank to pay $50,000 in cash if he or she wants to, but can't afford a $926/month car payment.
Edit: Can 0% interest loans be a trap? Most definitely. I just want to be clear I was talking about a particular scenario when they can be used effectively and safely.