I doubt states would be concerned about taxing relocation benefits specifically because it might dissuade people from moving to the state. It would be a tiny tax liability relative to other tax liabilities that already vary such as income tax, property tax, and sales taxes. If those differences do not dissuade people from moving, then a one time ~5% income tax on $5k to $20k of moving expenses is not going to move the needle.

Yes, I'm referring to US federal income. No idea how state income tax rules work, as there's so many different rules and I'm sure they vary (e.g., one state may allow relo deductions, one state may not).