Roots will grow to the best water supply available, so your drainage line is a prime target if it has relatively constant flow of water. If you restrict root growth you limit the total plant growth and health. So you be the judge.

I just planted one Saturday and although I cannot directly answer your question, I am wondering how large your planter is. I am under the understanding that the Arbutus marina gets anywhere from 25-30 feet tall, although I have gotten alot of conflicting info.

Some other things I learned in my research:
The roots do not like being disturbed. so if you are planting other things with it... I am guessing they should be long term plants.
Water deeply to establish and develop DEEP root system (I am thinking you would not want to restrict)
Arbutus Unedo is a smaller tree, but I think it has dif. bark.

In relation to what petzold said...In my community, they require (2) trees to be planted in every front yard. Our previous owners went overboard and planted alot of trees. We have had alot of problems with tree roots in the main line and have had to have it routed out about 1x per year. The plumbers say it is a common problem in our community because of the tree requirement.