We're not likely to find a time in the history of electoral politics that didn't feature at least one of the candidates involved taking shots at each other. We'd also be hard pressed to find a case when all parties in an election managed to resist the temptation to angle those shots towards matters that have nothing to do with public policy.
After all, the leadership of a nation presents as many opportunities as it does stressful responsibilities. And whether a politician wants those opportunities for the people or for themselves, it's generally expected that they'll use whatever weapon they can to increase their chances of gaining them.
However, that doesn't mean that every candidate will take every opportunity to score some political points. Sometimes people will simply have to do what they can live with and that seems to be what's stopping former Vice President Joe Biden from exploiting one particular subject in his campaign.