Microsoft Upsizing Wizard upsizes Access byte field to SQL Server bit column with 'Required' property set to False.
It is a bug in Microsoft Access Upsizing Wizard 97/2000/2002/2003.
Before upsizing to SQL Server 2000/2005.
After upsizing to SQL Server 2000/2005 and linking it back to Access
There is nothing you can do apart from fixing it manually after upsizing in SQL Server.
Set property 'Allow Nulls' to 'No'