Workers (or their union representatives) asking for wage increases are usually told "sorry, there's no money". If the company then does a share buyback, employees who haven't seen significant raises in years are going to take umbrage. In that context, that's why buybacks are such an issue: they're seen as a "fuck you" to underpaid workers.
Clearly, companies spend money on things (offices, factories, private jets, etc) other than salary increases all the time. The company has decided that its money is better spent on this thing rather than the other. Sure, workers might quibble about whether the money is better spent going to them but buybacks are not any different than any other thing the company is spending money on so it is not any more of a "fuck you" than other types of expenditure.
Companies generally pay their workers more only if they have to (tight labor market, threat of stoppages, etc). Companies generally do stock buybacks if that is the best use of the cash at hand (ie there are no better investment opportunities for the capital available). The considerations are completely different.