Going strictly on what information was in the article...
I believe that the word "if" was used as part of an "if - then" scenario, ie., if they ____ then this happens.
It is a disruption when you have kids walking into the classroom after the rest of the group has gotten started with whatever it is that they are doing. Not all schools do breakfast in the classroom. When a child walks in late, all the kids look to see who has entered the classroom and the attention focus is lost. The latecomer needs to put the coat & backpack away & then get to their seat, more opportunity for loss of focus. The teacher must then re-focus the attention of the group on the daily lesson, so the late arrival impacts the entire class. The child arriving late is playing catch-up and may have missed directions or other important information for the beginning of the day. The late arrival may interrupt the morning announcements (via PA or broadcast) in which case you have students and teacher missing important information for that day.
I've done the herding cats routine (with 4 kids, not 3) and 6 of us trying to get to the one bathroom at the same time. My kids were NEVER late to school. I may have been late to work (designated cat-herder) a few times but I had understanding employers and my arrivals could be flexible on most days due to my schedules.
By the time it gets to this stage, the school has already attempted to work with the parents -- talking to the kids, sending notes home, calling the parents, sending more letters home, calling the parents, involving the guidance counselor and maybe a pupil personnel worker (school social worker), more phone calls. Quite frankly, it gets to the point that so much extra time is being spent on contacting these parents that it really becomes a time/resource/disruption issue for the school.
Why should these two parents be allowed to take up so much of the school's valuable time and negatively impact so many other people (3 classrooms, 3 teachers, Principal, Assistant Principal, Secretary, Guidance Counselor, higher-up administrative staff, etc. -- close to 100 folks right there!)?
If these folks can't be responsible enough to get their kids to school on time, then perhaps they should teach them at home. I'd love to hear what a judge would say if that lawyer happened to be late for court...