It makes sense because you didn't pay federal taxes on that amount last year. Ideally your withholding should have been perfect last year and your refund would have been $0, and then you would have paid the federal tax amount on it last year.

Relatedly, I got a ~$5k state refund last year and I'm not paying additional federal tax on it this year because the SALT deductions are now gone under Trump's tax changes, so my state tax burden doesn't affect my federal tax burden anymore.

Into the weeds a bit here, but if you deducted your payments to the state in 2017 on your federal 2017 tax return (filed in 2018), and you got a refund from the state in 2018, you should be entering that on your federal 2018 tax return.

Since you won't be deducing SALT in 2018, any state refund in 2019 wouldn't be reportable for federal tax year 2019.

If this were otherwise, people would have purposefully massively over pay to the state in 2017, claim a large deduction in tax year 2017, get a large refund in 2018, and not have to pay back the deduction they didn't really deserve.

Yup. I happen to be in an AMT situation where the state refund from last year isn't taxable this year, but it might still be for others as you point out.