They are? When one thinks about a 1st liner, does anyone say "we desperately need to add a 50 point guy to our sagging offense" or do they talk about guys who are 70+ point guys?
Guys like Bunting (and Rakell, and Tatar, and Nyquist, and DeBrusk) are middle six players. They can play on a top line, but only as the third option with two legitimate top line players. But they can never BE the top line player on a line.
That's not what being middle-six means for Bunting
Being middle-six means there's either 1 or 2 better left wingers on a team than you, on average, throughout the league.
That is not the case with Bunting at all.
There were 29 left wings who reached 50 last year. He got 55, not even with optimal usage or linemates. Even if you account for injuries, at the very worst he's a high-end 2LW in his down year.
Only reason you'd put a guy like that on L3 is if you're either stupid, outrageously stacked at LW, or are looking for a tactical advantage like with Kessel with HBK.