That would be as foolish as assuming he's guilty. Are you one of those that trusts others until shown a reason otherwise?
It is no more foolish than presuming he is innocent.
It is the same thing.
If we have no reason to think someone is innocent, then we should not assume innocence.
If we have no reason to think someone is guilty, then we should not assume guilt.
If we have no reason to think either way, then we should not assume either way.
Many people, however, will assume innocence when there is no reason to suspect guilt. That's a problem.
Likewise, many people will assume guilt without good reason, and that's a problem.
All of the above has to do with assumptions without good reason.
If I have good reason to suspect innocence, then what I will do will not be to assume innocence but rather to presume innocence--kind of like a cautiously held belief subject to change should prevailing counter evidence surface to think otherwise.
Same thing with a good reason to suspect guilt. If I have good reason to base a belief of guilt, my view ought not be characterized as an assumption but rather a presumption based on reason.
The basis for my presumption that someone is (oh say) innocent will not be for the same reason the justice system should do so. We, as a society, value freedom, and in there, an argument can be made to reflect the notion that there is good reason born of principle to presume innocence in a court of law. If I personally presume the person is innocent, the basis will be predicated on factors that are weighed against each other--an entirely different basis than why courts make the presumption of innocence.
If I (personally) presume that someone is guilty; hence, if I hold the position based on reason, then do not make the claim that I'm making an assumption with no reason for thinking as I do; instead, understand that I'm presuming (until shown otherwise) and with reason that the person is guilty.
It's okay for me to presume guilt while a court justifiably presumes innocence.
Person assumes innocence: not good
Court assumes innocence: not good
Person assumes guilt: not good
Court assumes guilt: not good
Person presumes innocence: acceptable with good reason
Person presumes guilt: acceptable with good reason
Court presumes guilt: not acceptable by today's standards
Court presumes innocence: acceptable (as there's good reason)