When an acronym (or initialism) becomes commonly used it gets applied to other things than the thing it originally meant. That's kind of a awkward way of saying it, but an example would be something like USB bus, where the B in USB means 'bus'. You can also have USB connectors, or USB cables so saying USB all by itself is kind of missing something. In this case I would maintain that the redundancy of USB bus adds something to the understandability of the term. The same is true of ATM machine, since you can have ATM cards. Other so-called pleonasms such as PIN number clear up any ambiguity with 'pin' or even, in some dialects, 'pen'.