He said "viable", not "used more than necessary". It's one thing for a build to be viable - say, taking Ignite instead of Smite on a Champion with strong innate sustain in the Jungle - but it's another thing altogether when an item is being used for more than its intended purposes.
Ninja Tabi on bottom laners would be okay - if it didn't cause problems or uninteresting gameplay. 'Cause, wouldn't it be fun if you decided one day to play bottom lane, and went up against a Caitlyn ADC who started with a Doran's Shield, rushed Ninja Tabi, was running Fleet Footwork + Overheal, and had a healing Support like Sona/Soraka?
Granted, it's unlikely to happen, but when you have people stacking defensive mechanics on what should be an offensive champion/role, to the point where there's not really much counterplay available except "all in and hope you win", the game becomes less fun.
And, bear in mind, I don't know if that's a potential angle Riot sees it from, but, still, from a player's perspective, it makes a ton of sense to nerf unduly safe builds/tactics.
Which - again, I have to reiterate, my example is a bit of an edge case, since most of the stuff I mentioned has been recently nerfed in defensive value and I genuinely know fuck-all about safe bottom lane builds, but I'm willing to wager that it's a plausible tactic that I'd genuinely hate to go against.