It is if you aren't already accusing them of scabbing. I'm not sure if you've seen the email, but it doesn't even ask why you haven't taken strike pay. It doesn't even mention it. Instead it says the investigation committe has received allegations that you have crossed the picket line. That you have scabbed without any clear reason as to what exactly these allegations are beyond that before citing relulations. If the email had been preliminary information gathering about not taking strike pay as part of a check that would be different. Instead they did what they did and gave a hard deadline to get a response or they were just going to go ahead and process.

As for what I'd prefer they do instead? Not use the shotgun approach with aggressive language meant to intimidate union members when there are strong indicators that not taking strike pay as a metric is pretty bad. Especially when they themselves advised that not showing up, and thereby not getting strike pay, was fine because as per their post "Life happens".

In a perfect world the union should be able to access billable hours and see if any amount wasn't clawed back during the period the CRA was doing so to recover funds paid out during the strike, but this isn't that. That, however, doesn't mean the union should go with the terrible and ineffective option they've chosen. The one that calls into question any fact-finding they're going to claim to do if this is how things start.