Because at some point their emails about the firing will be subpoenaed, and if they lie about it now it's going to bite them harder later.

They still didn't have to outright confess to the crime though. When talking to journalists you're allowed to say "no comment".

That's why we got training at Google that more or less told us not to put these kinds of things in writing.

Remember: the e in e-mail stands for evidence.