Because it doesn't fall under any current category.
When your support takes 
it's trolling. But they aren't inting, they're not afk, not toxic in chat, and not using hate speech. Riot, please just add a "Trolling" option under report.
Thats what MF support is supposed to build...
http://www.probuilds.net/guide/show/OCE/174223890/563169 http://www.probuilds.net/guide/show/NA/2536000095/59189004 http://www.probuilds.net/guide/show/NA/2498241812/72411076
You want someone banned for following most optimal and META build for MF support?
Isn't there a griefing option?
Edit: Also like archer said, that isn't trolling. It's a legitimate pick and build.
Hello people?? What trolling? What griefing? Guy literally built everything he is supposed to as MF support
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2549294535/205691626?tab=overview
He built Sightstone first, he placed most wards, he got first blood, he won his lane, he placed 7 control wards, he didnt die a lot, he did decent damage.
Hell if someone should be punished for griefing its OP with his 130 CS as mid laner in 33 mins (thats less than 4 CS per minute)...
Negative behavior/harassment
The categories are for riots internal use. They dont really matter to you. The automated review is the same in any case.
Trolling is defined in many different ways. To your opinion, that is trolling. But that does not apply to everyone.
I could run 
in the bot lane with the intent to build 
in optimal order (whatever that is). Sure I can run with 
but you know I don't want to invest in a periodic situational summoner spell. I'd rather combat and/or track down.
If I were reported for trolling, what would be the trolling? The smite, the item build, or just the champion even if I explain my purpose? There's too many variables to account for and you, any other player, and probably even Riot cannot fully dictate what classifies it as a punishable option.
Except, what you described ISNT trolling, and also isn't reportable. unclench your butthole and try a little build variance once in awhile
