How cast through a staff use your ability mod?

@JeremyECrawford Quick followup question: If you personally aren't casting the spell through a staff, how can it use your ability mod?

— Kevin Kost (@kpkost) August 27, 2016

If a magic item tells you to use your spellcasting ability, you do. If it doesn't, you don't (see DMG, 141). #DnD https://t.co/0Suc7v2lzT

— Jeremy Crawford (@JeremyECrawford) August 27, 2016