The riders - through their teams - are employed to do racing. Employees generally have a right to have their voices heard.
But as you yourself have said, there are plenty of other races they can go to. Road cycling is an outdoor profession, and weather is something that happens outdoors.
My problem is more that organisers have become so weak in how they deal with the péloton that the riders can just basically down tools any time they like and the organisers just roll over and say "please sir, can I have another?" Whether it be Vegni agreeing to let riders not ride the full course because "well, it's a bit rainy, and in retrospect maybe this stage didn't need to be so long", or the bunch in the 2016 Vuelta going, "you know what, there's enough of us here that they'll never bounce us from the race, let's just get off and walk" and Guillén being too weak to stick to his guns and have a reduced péloton in week 3, legitimising the riders ignoring the time cut, riders have found it increasingly easy to find support for protesting conditions, leading to some absolute farces as organisers scramble to find solutions. From that joke at the Gran Camiño race where the riders decided that the conditions were too bad, when the break had already got through the worst of it and onto some perfectly safe roads after, but the stage got annulled because the bunch decided to clamber off, to stage 2 here where the actual finish was perfectly safe but the riders protested because of the previous day's disaster and got the finish moved to somewhere more dangerous than the real finish, then soft-pedalled it like Cancellara was still active, now we have riders self-declaring conditions too dangerous any time they don't fancy it. Yes, the onus is on the organisers to provide a safe stage, but this now makes twice in the Vuelta where a stage has been amended while running 'for safety reasons' annulling a finale that was, in actuality, totally safe to run. And we know this because they allowed the fight for the stage win to continue. We've seen a few stages where this 'GC is taken at X distance, but race is still live for the stage' format happens, but typically it's been in flat stages so it's just been about the sprint, and it hasn't been so egregious watching the GC contenders cruising at cyclotourist pace.
I suppose one problem is that the Premier League-ification of the sport with the WT being increasingly locked off (even before the UCI got scared of Sylvain Adams suing them and locked off half the "wildcards") means that a lot of the invites are compulsory. WT teams can't protest a race they don't want to do if they don't like the organisers because the WT licence is contingent on them showing up - but simultaneously the organisers can't un-invite a team that they feel has disrespected the race and invite somebody else instead. You know, like a couple of the Flemish semi-classics did to BMC in 2010 where they revoked the invite on learning BMC would not send Evans, Ballan, Hincapie or any of their name value riders, or Direct Energies being kept off the Vuelta invite list for three years 2017-19 after their entire team missed the time cut in the Formigal stage.