Paid relocation expenses is fairly common outside of SV for highly compensated employees, however, they are structured so that you must pay it back if you don't stay employed for a certain time (usually a year). I've never heard of a relocation bonus that wasn't structured this way.

Retention bonuses and starting bonuses are structured the same, you sign an agreement to get that bonus that says you pay it back if either you or the company decides to end employment within a certain timeframe.