Oregon Senate Bill 5, for all intent and purposes, is basically a jock tax. It was created and passed when the Expos were threatening to move, and there was a need to show some kind of source of for funding of either building a new stadium, or renovating PGE/Providence.
It's probably the best thing that came out of that version of the "Bring MLB to PDX" movement.
The reason it is "going before the legislature" now is that they want to update it. Why update it? Well, it's based on 2005 (ish) salaries. Salaries have skyrocketed, and adjusting it to what the salaries would be today more accurately shows how much they could capitalize on the taxes created from the players and people associated with a new franchise (which would cover the bond iirc).
Also, this is only going to happen IF a team is moved here or an expansion franchise is granted to the city (etc). You, as a tax payer, won't pay a new tax either. The only increase you might see is maybe in ticket prices or merchandise bought from the team. But if you never go to a game or go to 81 games a year, this modification of Senate Bill 5, will not increase your tax burden.