I heard some silly stuff in my life, but this takes the cake. You walk into a place of business, they have an item priced at $1.99, you pay that and go home. There are no ethical issues. It is simple supply and demand. One of the biggest draws to getting people into thrift stores is the possibility of a bargain. The idea that someone is being unethical for patronizing these stores and paying the labeled price is simply preposterous.
It does keep stuff out of the landfill, provides the poor with a source of decent clothing, especially the working poor, creates a lot of jobs and supports a number of organizations that provide services for the blind, developmentally disabled, and so on.
Keep in mind these are not small storefront operations run by volunteers supporting PAWS or similar organizations. I'm talking about 75,000 square foot stores in old grocery store locations with six check-out lanes and a large parking lot. These are multi-million dollar operations with 20+ employees each . In particular, Value Village (same owner as Savers) is a for-profit business that buys donations from NPO's by the pound and sells them in their stores. The one in the Capitol Hill district in Seattle occupies the old REI headquarters store and has three floors of merchandise.
And there is a side of service that goes with it-- waiting in lines that would be considered too long in "regular" stores, cashiers that are always in training, no warranty, no cash refunds, or as-is sales, and the items can be perfect or have all kinds of flaws. So you are inspecting everything with the idea that it is there for a reason and you need to discover what that reason is. You could get home and find it doesn't work, has a flakey zipper, a hole you didn't notice, smells like the cat used it for a litter box, or is a counterfeit (North Face is rampant). There is risk and reduced service that balances the possibility of a bargain.