What's to stop them? The law.
What's to stop me from committing any crime? No law can prevent anyone from deliberately committing a crime. It can only provide a venue of punishment and redress.
If you read the fine print, this website tells how entities get on the list.
1. They gave through a political action committee sponsored by the organization (This is not illegal. There is a difference between donating to a PAC and to the candidate directly.)
or
2. individuals connected with the organization contributed directly to the candidate.
Again, nothing illegal as long as the individuals are US citizens. There is a difference between tallying up individual donations and sorting them by employer and the amount the employer donates. This is one of the tricks these "monitoring" organizations do.
Consider this: If somehow it could be determined that members of the military donated megabucks to a candidate, does that mean that the DoD donated megabucks to the candidate? Of course not.
Donations from a corporation to a PAC need to be counted separately than donations from individual employees of that corporation. Like counting coconuts and cinder blocks.
To lump them together, gives an inaccurate picture.