Possible, but I also guess the reason was along the likes of "I want to be able to send and receive 100MB powerpoint slide decks, but central IT has a 5MB cap"... this one is something I regularly hit with clients back in ye olde freelance time. Record low was a client with 1MB attachment cap and 100MB of quota.
How do you know this? The Members themselves are best placed to determine the type and scope of this. Not all MPs will share and delegate, and not all interns will have such facilities extended to them.
The prescribed way is to have staff undergo vetting, then be assigned network privileges (with whichever delegated rights are specified). But this is the real world, where we know that trusted individuals in teams share credentials.
None of this precludes MPs (at least, those not on Government payroll) from setting up and using third-party accounts for their sole use - which is exactly what MPs do.
There wasn't an original offence, there has been an allegation which has been strongly denied.
The refutation alone, that nobody would know who had downloaded porn even if it had happened, implies that this is a shared network account and it was being used to log in to desktop sessions.