This sentence is vague. I wonder who is colonizing and where the natives went to avoid the colonizers. In any case, I am guessing that "dedurre" in this context means "sottrarre" ossia, "remove". The natives were "removed" by the colonizers. Yikes!

deduzione= transfer

...
taking the lands from their legitimate owners and giving it to the (Roman) soldiers/poor people
...
or
...
establishment of colonies/plantations (on already existing properties)



please see

"Gaius' agrarian reforms continued his brothers' policies and proposed a scheme for the establishment of colonies which would provide resettlement opportunites for the landless urban poor. He also passed laws requiring the Senate to reform its military recruitment practices (which had become incredibly oppressive under the manpower requirements of the expanding empire). He also instituted a program which required the government to guarantee a maximum grain price. This meant that urban citizens wouldn't be ruined by fluctuations (inevitable and manufactured) in the price of imported grain (since going to latifundia, Rome had become dependent on imported grain)"
http://abacus.bates.edu/~mimber/Rciv/2nd.cen2.htm

"The second mode of alienation was that by assignation. Lands thus assigned were known as agri dati assignati. The gift on the part of the state was gratuitous, and ownership passed wholly to the assignee. The land so given was definitely surveyed, marked out and registered. Such an assignment might take one of two possible forms. It might be the means of establishing a new "plantation" (colonia), with some independent political organization of its own, however slight - a settlement, therefore, which could be thought of as an entity separate from the city of Rome and from any other municipality"
http://www.1911encyclopedia.org/Agrarian_Laws

"La deduzione di colonie (deductio = trasferimento vero e proprio di coloni)
Versione originale
Riassunto: La fondazione di una colonia era un'operazione di carattere militare, i cui costi rientravano nel bilancio dell'esercito. Per diventare colono bisognava perciò arruolarsi. I coloni ricevevano un contributo per le spese di viaggio e di primo impianto. Le fonti antiche sottolineano la funzione di difesa e di presidio delle colonie. Gli abitanti erano divisi in tre classi: gli equites = cavalieri, i pedites = fanti ed i centuriones = centurioni. La divisione era fatta in base al censo. La distinzione in classi valeva anche per le assegnazioni di terra, che non avvenivano in misura uguale. Alle classi meno abbienti non venivano assegnati grossi lotti di terreno poiché non avevano i mezzi sufficienti per la coltivazione e la gestione. Al ceto dirigente, che poteva servirsi di manodopera indigena, venivano destinati lotti di maggiori dimensioni. "
http://digilander.libero.it/imiani/Romagna_preromana/sottome...
"Inizia così per Roma la fase 'espansiva': con l'aiuto delle città-stato latine, di cui si pone a capo, la città difatti si annette nuovi territori, sconfiggendo i Volsci nel 431 (dando inizio inoltre alla pratica della deduzione delle colonie, pratica consistente nel trasferire in veste di coloni alcuni dei propri abitanti - i quali, almeno in questo primo periodo, non necessariamente sono cittadini disagiati - sui territori occupati)."
http://www.homolaicus.com/storia/antica/roma/roma_repubblica...


it's a legal matter, not only a socioeconomic oppurtunistic choice
perhaps you could find interesting (I did!) giving a look to these sites also

http://www.jstor.org/pss/4436225
http://it.wikipedia.org/wiki/Discussioni_utente:Lo_Scaligero...
http://books.google.com/books?id=D_v8Fl5dvvUC&pg=PA96&lpg=PA...
http://www.parmaromana.eu/la_centuriazione.htm
http://www.scanzo.altervista.org/testi_04_05/catone.pdf
http://www.dirittoestoria.it/iusantiquum/articles/Sacchi-Ter...
http://spazioinwind.libero.it/popoli_antichi/Italici/Falerii...

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Note added at 1 day1 hr (2009-11-28 00:48:34 GMT)
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thank you!
(after seeing your note to the others' answers I have to confess that) I searched thoroughly, again and again, to found a standard English term for it but... it seems it does not exist! sorry
I guess that it could be due to the fact the 'deduzione coloniaria' is actually a quite generalist expression that does not distinguish the different steps of the Roman empire evolution. Instead, English texts define precisely these different phases with their real name (e.g. agrarian laws), so giving more importance to their causes and context than to their effects description (as this Italian expression does)...