I don't know why they don't distinguish entrants who are fund-raising versus time-based qualifiers.
If you ran a marathon sufficiently fast to get under the qualifying time, that doesn't mean that you are required to run THAT FAST on race day for the Boston Marathon.
Some runners try and run fast on race day and discover that their body has hit the "wall" and are forced to run slower, walk for much of the remainder of the race, or worst case, drop out.
Qualifying times for the Boston Marathon are different for older age groups. Older runners can get into Boston with a much slower time than someone in their 20s.