This has been done in the past and there are people who would like to try this again. Imputed income is money that you never saw but they claim that you received the benefit of. To give an example, there was an attempt in the early 1990's by the US Congress to chrage people for imputed income on their mortgages. If the taxing authority (IRS) determined that your house payment was less than what the going rate for rent in your neighborhood was, they imputed that the difference was income on which you would be taxed. To put numbers to this, if you have $1000 a month house payments and houses in your area lease for $1500 a month you have an imputed income of $500 a month. At the end of the year they tag an additional, imaginary $6000 on your annual income on which they want real money paid for taxes. This idea could be applied to sales/use taxes for goods received.
Now move this to software. If the MSRP of MS Office is $600 (I don't have an exact number) then you are imputed to have earned $600 if you download OpenOffice. Therefore you own sales tax on $600 dollars even though you didn't pay any money. That would be about $50 here in Texas that you owe.
They can dream up about any way to gain tax revenue.