Some offices give employees time off during holidays, but others don't. Some even make employees come to work on Christmas Day or during other festive seasons.
It's not as simple as that. It depends on the nature of work the company is doing.
Even I am myself a salaried employee but my company doesn't force us to work during holidays.
Ofcourse, there should be 2-3 people at least to support during the holidays but the rest of the people can take their time off.
But I believe that cannot happen with every company. Just an example, food shops, grocery stores etc... have employees working during the holidays too.
If they don't operate during the holidays then how would the rest of the people enjoy their holidays.
Your suggesyion of making it voluntary is valid though and should not forcer everyone to work during holidays but on the other hand if none of the employees are working on holidays then they will surely tell some of them to work.