I'm not sure that anybody is against the ruling itself, but more that a company would decide to institute that policy in the first place. It was well within Mylan's right[0] to raise the price of EpiPens an egregious amount for no real reason other than to increase profit, just as it's well within Monsanto's right to have you sign an agreement prohibiting you from using new seeds for re-planting, but that doesn't mean people aren't pissed off about those business practices.

Fundamentally yes, some are also angered by the fact that current IP law / court rulings allow those types of agreements to be legal, and there's potentially an argument / opportunity to change the law to prohibit that type of agreement, but primarily it's just a BS move by a company that makes people mad. To your music point, many also consider it a BS move to prohibit saving the music you "buy" on iTunes to more than N devices. It's completely legal, and you definitely agreed to those terms when you bought it, but that doesn't make it any less infuriating.

[0] However they may have gotten in hot water for that being considered anti-competitive via deals they signed with public schools. https://www.theguardian.com/business/2016/sep/06/epipen-pric...