Not a scientist either, but my guess is they wanted truly compare the viruses. Knowing the mice are susceptible it eliminates the possibility that they didn’t catch it in the experiment to see how the virus impacts the whole group for an accurate result.

Boston University replied to these claims. Seems like poor journalism and the media taking things out of context.

From BU’s response;

“The animal model that was used was a particular type of mouse that is highly susceptible, and 80 to 100 percent of the infected mice succumb to disease from the original strain, the so-called Washington strain,” says Corley. “Whereas Omicron causes a very mild disease in these animals.”

That 80 percent number is what the media reports latched onto, misrepresenting the study and its goals

Here’s a link to their response article.

(https://www.bu.edu/articles/2022/neidl-researchers-refute-uk-article-about-covid-strain/)

Edit: Just wanted to point out it’s not an official response from BU, it’s by The Brink Staff. I’m guessing they have some involvement with the University being their article is on Bu.edu.