I'm not religious but I'm interested in religion.

You are applying the Protestant framework (I can save myself) to Catholicism (only through the church can I be saved) hence why you invoke Bible as a final arbiter of what is and isn't allowed.

In the old churches (Catholic, Orthodox etc) "Canon" is a combination of early church fathers writings, tradition, the bible and pronunciations of the current head of the church. Furthermore in Catholicism the bible is largely treated as allegorical, not literal.

Now since you are definitionally a sinner, and since salvation can only be obtained through the intermediaries of God: the church, being part of the church mysteries (baptism, communion, prayer, confession etc) is far more important than following any specific part of the bible.

Praying to Saints is not considered a sin since you are under no misapprehension that you are praying to a god, or that they are anything more than humans that lived an exemplary life and are amongst God's favourite children. It's like asking the local lord for a favour - you are not confusing him with the king.

It's a marketing thing.