New laws shouldn't, in general, be retroactive. The fact that a grandfather clause is necessary to specify that a law isn't intended to be retroactive is bothersome.
Certainly not. It's all based on the purchase/exercise/offering date. Let's take it to the extreme: if homicide had been legal, and society decided we needed to outlaw it, it stands to reason that homicide committed before the law was enacted would not be prosecuted, but only those cases happening one or after that date.
Tax law isn't so clear. There's when you were granted the shares, when you acquired them (which may or may not be a taxable event, depending, among other things, on whether there's a spread between strike price and FMV), when you sold them (which is a taxable event), and when the law was enacted. There are probably yet other subtleties beyond those.
The grandfather clause covers the case when the enactment date falls amongst the others. ISOs purchased before the enactment but sold after. ISOs granted before the enactment, but purchased after. Double-triggers. Are you sure you know how the law applies, and what your tax liability is, without explicit statute to that effect, in all of those cases — or others I haven't listed, or even imagined? Is your accountant? Are you willing to bet an audit on that?
EDIT: phrasing
Criminal law absolutely recognizes that causal chains have a "first link", without which the rest of the chain wouldn't even have happened. It also specifically subjects the party causal to that first link to special scrutiny.
It is, therefore, the exact opposite of retroactive.
EDIT: phrasing.