>> costs of compliance for a full-time employee

I'm not sure this is the driver. Instead, the primary factor is likely the fact that hourly employees cost a 50% more the moment they cross 40 hours in a week.

>> Google would rather hire 15 guys at 40-60 hours instead of 40 guys at 20 hours

Not if they are paid hourly, they wouldn't. It would just be throwing money away.

In my way-back days working hourly jobs in retail/fast food/etc., we would game this by trying to get as many excess hours as possible early in the week, so that we could get to 40+ hours by the 5th/6th day. Then, we'd aggressively try to get extra shifts from people who called in sick, etc. But management always tried to prevent this.