Pay attention, if you click on details button - product category will be also shown on another webdynpro.
Alternative, you can try to assign custom handler class for CATEGORY_ID field in SPRO->SAP Supplier Relationship Management->SRM Server->Cross-Application Basic Settings->Extensions and Field Control (Personalization)->Configure Field Control->Configure Control for Fields on Item Level
more info here: Field configuration in SRM 7.0 - Supplier Relationship Management - SCN Wiki