Tax on food to go

So I?ve been on both sides of the fence.  When I order food to go, some places charge me tax only when the item is heated.  Say subway, if I order a sandwich to go, and I get it toasted, then tax is applied.  If the sandwich is cold, there is no tax.  But I?ve also been charged tax on cold items to go and when I asked about it, associate said tax is always applied to items even when it?s cold as long as it?s protein.  Not sure that makes sense.  Technically a cold cut is protein but there?s no tax on it because it?s a cold sandwich.  Just wondering what the rules are.