There are two manners to dispose of issurei hana'ah: through burning and through burial. The difference between these methods is that ashes of items which must be burned become permitted after the burning process is complete, but items requiring burial remain prohibited forever. The reason for the distinction is explained by Tosfos (Temurah 33b): once the Torah command to burn an item has been fulfilled, the mitzvah of disposal is finished and there is nothing more the Torah requires. Items that do not require burning have no proscribed method of disposal and therefore the task of eliminating them is never absolutely completed, hence these items never become permitted.
The Rambam paskens (Psulei haMukdashin ch 19 based on Temurah 34):
כל הנשרפין, לא ייקברו. וכן כל הנקברין, לא יישרפו: שאף על פי שהוא מחמיר בשריפתן, הרי הקל באפרן--שאפר הנקברין אסור.
While burning an item that requires only burial may seem like a great idea in that the disposal is more thorough, in actuality this may lead to mistakingly thinking the ashes are permitted. Therefore, only items which must be destroyed by burning should be burned and all other items should be disposed of in the appropriate way.
The Achronim point out that there is one Rambam that seems to contradict this rule of thumb. Writing in Hil. Chameitz u'Matazah (ch 3) the Rambam paskens:
כיצד ביעור חמץ: שורפו, או פורר וזורה לרוח, או זורקו לים; ואם היה החמץ קשה, ואין הים מחתכו במהרה--הרי זה מפררו, ואחר כך זורקו לים.
The Rambam clearly rules like the Chachamim, who allow chameitz to be disposed of by any means, and not like R' Yehudah, who required burning it. Yet, the Rambam still allows for chameitz to be burned. Why in this case is the Rambam unconcerned for the improper (and prohibited) use of the ashes of chameitz which have been burned? Why does chameitz not follow the general rule that items which do not require burning should not be burned?