You are right that the distribution is a return of capital; hence, it is tax deferred. This is a come-on bait of the MLP. Also, you are right that the UBTI is taxed if it excesses $1,000 per account.
But..... remember this if MLP stock(s)in your taxable account:
1. While you hold you MLPs stock(s), your partnership interest, items 1 thru 11 of your K-1 form, is taxable.
2. The real whammy is when you sell your MLP stock(s), you have to pay taxes on ordinary gain, partnership interest, and capital gain. If you hold your MLP stocks too long, you may need vasaline ointment for your rear end.