The pack size is a legal restriction, not the number of packs you can buy in one transaction as long as the total number of tablets does not exceed 100 (i.e. legal restriction is in effect 6 packs in one transaction), from what I understand, while the 2-pack restriction is a "voluntary best practice".

Those 'voluntary' limitations, and there are others, are a bit of a pet peeve of mine because they strike me as hypocrisy/posturing/paternalism.