Exhibit A is the fact that the target was innocent civilians at a race, not a government facility.
Even Timothy McVeigh didn’t target the general public. Not that he seemed to be much concerned about collateral damage. Most terrorists are pretty clear about their intended targets, if only after the fact. These bombs weren’t set in the front door of the Boston federal building.
The only terrorists I am aware of who consider the general public including children to be their legitimate intended targets are the practitioners of the Religion of Peace (TM).
Eric Michael Rudolph is one of the few non-muslim terrorists, if not the only, I can think of who targeted civilians (excluding the civil rights era).
Exhibit B is the fact that the bomber or bombers unknown waited until all the VIPs that were there for the first-to cross-the-finish-line photo-op had safely gone on to the next photo-op.
McVey was a useful idiot for a Muslim group.