this and this
The issue here is that they had pace groups for 7.5 hour runners, they were under the time cut off, and these slower runners were treated differently than other runners by race staff and contractors. (The 2020 London Marathon has a time cut off of 9 hours. )
So the event is comping their entries to make up for it.
I would also expect a marathon event to offer free entries to everyone under 2:30 if those faster runners had problems with aid stations not being set up when they arrived, the course was not yet clearly marked, and/or those working the event had told those faster runners to 'slow down' or 'stop and wait till we are set up and ready', or told them they were too skinny.
If they are going to allow 9 hours to finish they should provide the same amenities and treatment to everyone who enters and completes the event in under 9 hours.
Whether or not they should allow people to finish in 9 hours is another thing altogether. I believe its actually harder and more costly for them to accommodate slower runners than faster runners, but if they want to do it that's ok.