3.25% is the annual interest rate, it compounds and will cost considerably more than a one-time 10% tax would.
Treasuries and certain other government securities count as "cash" in most cases.
Edit: Apple reports most of the amount bandied about as their "cash" as being in "long-term marketable securities." [2]. So it's a mix that is definitely not designed to all be instantly convertible to USD with no penalty. I'd guess most other large companies manage their cash similarly.
1 - https://en.wikipedia.org/wiki/Cash_and_cash_equivalents#Comp...
2 - https://www.apple.com/newsroom/pdfs/Q2FY17ConsolidatedFinanc...
Also, as ryonwaggoner points out, they are free to buy e.g. Treasuries with the offshore cash so that the effective interest rate closer to 1% (before the effect of the reduction in tax liability).