As much as baseball fans may get outraged when a hitter gets suspended for using performance-enhancing drugs, imagine how a (non-PED-using) pitcher feels.
Pitchers put in an incredible amount of work to develop their stuff and strike batters out. When an opponent tries to beat the system and takes PEDs, it makes a pitcher's job even tougher. And for pitchers without much job security, a home run here and there can see them reassigned to the minor leagues.
Guthrie has never been an ace by any means, but he has been more than serviceable throughout his career. He has put in a lot of work to stick in the majors, so when he sees someone benefiting from using a banned substance, it gets under his skin. After all, he has to go head-to-head with those players, and his contract status can be greatly affected as a result.
Just look at reliever Layne Somsen to see what can happen to a young pitcher facing a PED user.
On May 16 (a week or two after Byrd tested positive, according to Ken Rosenthal of Fox Sports), the 26-year-old Somsen—then with the Cincinnati Reds—got taken deep by Byrd in his second big-league game. Somsen was optioned to the minors two days later and put on waivers, where he would be claimed by the New York Yankees.
But it's not just the younger players who are affected. Former MLB pitcher Dan Haren certainly wishes he could take Byrd's at-bats against him out of his career numbers:
Detroit Tigers right-hander Justin Verlander was also irked by the latest suspension:
Byrd, as Rosenthal notes, has made $38 million in his career. One could make an argument that PEDs have helped him make the majority of that money, especially in the later stages of his career.
While he may have benefited from using banned substances, other major league players, like Somsen, have been negatively affected. And that's why many players have taken strong stances against PEDs in recent years.