No one is saying he can't have that view. The reason in this case why people want politicians to fall into an either or category is because 1)Brooker in the middle stance doesn't make any sense and 2) Voting is universal right either you believe everyone should have the vote or you don't, the same goes for the voting rights of prisoners.
But there plenty of political topics that don't fall into either or category - Taxes, Housing Healthcare(Although it really should be either or category), policing etc.
Oh yeah I wasn't trying to say you agree with Booker, if it came across like that my bad. But just to say again the reason Booker looks like he being forced into picking a side in the debate around voting rights for prisoners is because his current stance makes no sense. And also as
@Eboue has said Booker is running for president, if he wanted to not be pressured on his view then he should of stayed out of politics.
Your right in that it's not like going faster than the speed of light, picking and choosing who takes part in democracy is something we can do in a democracy and is something we have already done, be it working class white men, women, people of colour etc.
And while maybe there is some perfect balance to be found in the future for prisoner voting rights(Although I really struggle to think of any). One of the many great things about universalism is the difficulty it causes for the most malicious ill-intentioned people(The Republican Party)to roll back rights.
Once you've drawn a line that says this type of prisoner doesn't get to vote then you've left the argument open for again the most malicious ill-intentioned people to argue the line should be lower and should cover more prisoners.