I don't wish to belabour the point, not least because I'm, not knowledgeable about this but I think that people entering the US under the visa waiver programme wouldn't be affected by this. According to this CBS report:
The new tax cuts and spending bill includes a $250 fee for visitors coming to the U.S. on nonimmigrant visas. Here's what to know.
www.cbsnews.com
There are no details about how and when the fee would be collected and refunded (which is typical of the Trump administration, grand announcements with no actual detail behind them) but reports say that the fee would be levied when the non-immigrant visa is issued. These visas are good for a single in/out and so it's clear when you've left the country.
For the visa waiver programme you can have multiple in/out over a two year period so it wouldn't be clear when the $250 could be refunded.