I would agree with you if it were not for the fact we are using a non-historical or non-corroborative method of examination.
Here's Matthew 27 according to my bible (NAB)
44:The revolutionaries who were crucified with him also kept abusing him in the same way.
Here's Mark 15:
Let the Messiah, the King of Israel, come down now from the cross that we may see and believe." Those who were crucified with him also kept abusing him
Here's Luke 23:
39 Now one of the criminals hanging there reviled Jesus, saying, "Are you not the Messiah? Save yourself and us."
40 The other, however, rebuking him, said in reply, "Have you no fear of God, for you are subject to the same condemnation?
So it seems to me that the issue is not that Luke specifically mentioned it, it's that the other two (Mark and Matthew) did not specify, they just said "those who were crucified with him." According to the legal perspective, the truth would be that two witnesses claim everyone (in this case it doesn't show number just all inclusive words) reviled him but one witness was more specific in that one disagreed with another, which is not a direct contradiction but rather a disagreement in specificity of accounts. In a non-historical perspective, you'd just go back and get more specific testimony from both other witnesses. But in this case, since it's historical we take the more specific account to be the most likely to be accurate but also take into consideration the number of witnesses, and assume that most of them reviled jesus but there's a chance one spoke against the others.
The reason is because the other witnesses gave less specific testimony, not saying when or how they saw everyone abusing Jesus. This means that they could have witnessed the first half of the event, and missed the part where someone rebuked them, or their vantage point didn't allow them to see/hear it.
My personal opinion, after all the legalese and examination, is that it doesn't really matter, you're supposed to make up your own mind either way. But the definition of a contradiction is tricky in this situation because of the differing levels of specificity.