IANAL, but I have extensively researched this issue.

If you sell Bitcoin for an altcoin, or the reverse, this is a taxable event. You need to figure the cash value of the coin you sold at the time of the transaction and declare that gain or loss.  This is not a popular opinion, but the IRS does not care about popular opinions.

The only alternative to paying tax on these transactions is to try to claim them as like kind exchanges, which allow you to defer any capital gain until you dispose of the crypto that you bought in the exchange. To claim this you need to file IRS form 8824. Nobody ever actually does this. If you don't, you have no leg to stand on if you don't declare your gains when you trade between cryptos. Plus, it is likely that the IRS will not allow crypto to crypto exchanges as like kind transactions.

When you spend crypto on real world goods/services, you have realized a capital gain/loss on your crypto at the value of the crypto you have spent at the time of the transaction. Again, not a popular opinion, and again, the IRS doesn't care.

I expect numerous threads on this forum in 2018/2019 from people who are astonished to have heard from the IRS that it wants a lot in taxes, interest, and penalties on their crypto transactions. It is just not realistic to imagine that the IRS will ignore this very significant source of revenue.

They have data from the exchanges. We know about Coinbase. It is likely that they have data from the other exchanges as well.