You notice that Unit of measure is not passed on to Purchase order from preceding document
SAP Business ByDesign
Reproducing the Issue
While creating Shopping cart or purchase request you may change the unit of measure.
If a Physical unit of measure is maintained as base unit (for example days)in product master data then you will also get various respective Unit of measures(like hours,months, years etc) to choose while creating shopping cart or Purchase request or purchase order etc.
If Trading unit of measure is maintained as base unit(for example each) in product master data then you will get only that unit of measure until unless you have maintained other quantity conversions in product master data.
Since you have maintained Unit of measure under purchasing tab of Product master data ,that is taken as default in any Purchase order as expected.
You can manually change the UOM in Purchase order.
An alternative option is to submit a influence request: https://influence.sap.com/sap/ino/#campaign/886
KBA , SRD-SRM-PRO , Purchase Requests and Orders , Problem