Had one of the players knocked this guy out, he'd be guilty of assault. The dumbass "fan" would sue him, the club, the ballpark and who knows who else and probably win. Players are not responsible for enforcing the rules, security and/or police have that resposibility.
Tasers are used to quickly gain control over a suspect while significantly reducing the risk to the officer. Tackling someone usually results in a major struggle where the suspect and officer are injured. As officers are much more likely to have more interactions with many suspects than any one suspect will have with police, giving the officer more force options is warranted.
Do tasers have risks? Of course, which is why it is called "less lethal" and not non-lethal. But it doesn't present risks any greater than physical force (tackling, compliance holds, batons, sprays, bean bag rounds, rubber projectiles, etc.). It does reduce the risk to the officer however, thus the reason it is the most widely used less-lethal option.
2010 E70 X5 35d
Alpine White on Black with Dark Bamboo trim
ZAP | ZCW | ZPP | ZPS | ZRC | ZTP | 322 | 328 | 330 | 386 | 4AZ | 4UB | 655 | 6FL | 6NF
2008 Ducati 1098S Red naturally....