Teams are restricted from trading away future first round draft picks in consecutive years. This is known as the "Ted Stepien Rule." Stepien owned the Cavs from 1980-83, and made a series of bad trades (such as the 1982 trade mentioned above) that cost the Cavs several years' first round picks. As a result of Stepien's ineptitude, teams are now prevented from making trades which might leave them without a first round pick in consecutive future years.
Detroit can trade their 2015 first round pick if they want to. I highly doubt they do though. The only player Boston has that would probably require it in a Monroe sign-and-trade is Rondo but I doubt the Pistons are interested in him (especially if it meant keeping Smith around to get him to re-sign).
