...Many other antibiotics are based on small molecules that interrupting some metabolic process of bacteria. Bacteria develop resistance by making enzymes that will break down these small molecules once inside the cell. This new method attacks the outside of the cell directly, not something inside the cell. It wouldn't be impossible for bacteria to develop immunity to this, but it would be comparably very improbable.
It may be less probable, but not improbable. Bacteria are so abundant, reproduce so fast, and can have such high mutation rates that probability is almost always on their side. They also can develop resistance in many ways other than "making enzymes." The likelihood of them developing resistance is proportional to the selection acting on them. If we put this stuff everywhere, selection will be very strong. They will develop resistance.
Most public domain software is free, at least at first glance.