its not reasonable to expect people to start looking up what the exchange rate was when they received their bitcoins just so they could buy a cup of coffee without breaking the law.
The same thing applies to a US tourist traveling overseas. You take $100 and exchange it for 300 pesos (an example no idea what the exchange rate is) and by the time you spend those 300 pesos they are worth $102.81. You "should" report that $2.81 as taxable income.
Does anyone do that? Does the IRS try to round up every single $0.37 in taxes owed? No. The reality is most people are not compliant with the tax code. It is asinine in its complexity. The average tax return has a half dozen errors or omissions. Now if you buy some Bitcoins and later buy a $40M yatch with those Bitcoins he probably should consult with a CPA on the capital gain impact.