It's prob cos its antibiotic spray and they are trying to reduce resistance issues by limiting access. The sprays you get from agri merchant might be the same colour but they cannot be the same thing as they cannot sell antibiotics legally, they are vet only, they are probably more generic purple sprays or iodine stuff
However given Terramycin/Alamycin are used for specific periodic minor ailments and ones owners are quite capable of diagnosing and treating, it does seem overkill.
My vet won't give us pen and strep unless they've been out recently but they do give me Alamycin tins happily as they know I'll only use it when needed.