Slow play is obviously not enforced on the regular PGA tour, but there have been notable instances on the European Tour, as well as the example Spuzz mentioned a few years back in the Masters, where slow play is penalized.
And although (unfortunately) Spieth is ultimately correct that it's not a big deal (because of the non-enforcement mindset on the regular tour) it's still a very poor example to make such a claim openly, especially considering that it's a young player who will likely be in the spotlight for several more years acknowledging as much.
It's not so much an indictment of one player's indifference, but more so Finchem's indifference and unwillingness to change it, and how that has morphed beyond just what happens in the weekly non-major events.