Where in Carmen, San Diego does it state that? The Cast the Spell action is what I determined was the trigger of the 'only Cantrips are allowed after casting a spell' regulation.
Using a spell scroll is still an Activate Object action...the specific rules for Spell Scrolls just overrides the action used for this to match the casting time of the spell.
While one could interpret using an item to cast a spell is both a use an Object Activation and Casting a Spell action combined. The rules do not explicitly spell that out, (to my knowledge).
If indeed this is the interpretation a table takes, then shouldn't such an assumption also apply to an SSI? Thus you shouldn't be able to use an SSI and cast a non Cantrip Bonus Action spell?
I like 5e, but I would love a re-write of the rules. Clarity was sacrificed on the purple stone altar dedicated to the Elder Elemental Eye, in this edition. I don't want a return to Pathfinder or 3e style rules straightjackets...but 6 years in "rulings not rules" isn't aging well.