St. Augustine comments on this very question in De Doctrina Christiana (3.16). He lays out a general principal of interpretation.
>Si praeceptiva locutio est aut flagitium aut facinus vetans, aut utilitatem aut beneficientiam iubens, non est figurata. Si autem flagitium aut facinus videtur iubere aut utilitatem et beneficentiam vetare, figurata est.
http://www.augustinus.it/latino/dottrina_cristiana/index2.htm
Translated:
>If a saying of precept forbids either a shamefulness or wickedness, our commands either what is sensible or kindness, then it is not figurative. If, on the other hand, it seems to comman shamefulness or wickedness, or to forbid what is sensible or kindness, then it is a figure.
This seems like a common-sense principle in general. Though it might be objected that, due to some failing on our part, something that contradicts our common sense in scripture may in fact be a genuine challenge to our beliefs. But I think more often than not, Augustine's principal holds.
On this particular verse.
>Scriptum est: Da misericordi, et ne suscipias peccatorem [Eccl. 12.4]. Posterior pars huius sententiae videtur vetare beneficentiam; ait enim: Ne suscipias peccatorem; intellegas ergo peccatorem figurate positum pro peccato, ut peccatum eius non suscipias.
>It is written, "give to the merciful, and do not receive the sinner." The latter part of this statement seems to forbid kindness, for it says, "do not entertain the sinner." Therefore, understand the sinner is used as a figure for sin, i.e. that you not receive his sin.
So I think "suscipiat" here can be taken in a couple senses. One is that we don't aid the sinner in his sin, whether by aiding the sinner to commit sin or even becoming an accomplice to his sin, but another is that we don't take up the sin for ourselves.
Thomas Aquinas also comments on this verse in the Summa Theologica, giving a similar opinion.
>We ought not to help a sinner as such, that is by encouraging him to sin, but as man, that is by supporting his nature.
http://www.newadvent.org/summa/3032.htm#article9
Another possible and more litetal interpretation is perhaps that this passage is giving practical advice, while Christ's preaching in the Gospel calls us to a higher standard of benevolence. If we personally involve ourselves too closely with bad people, we may find ourselves the victim of their evil actions. On the other hand, we should try to render kindness and basic material needs to all people regardless of whether we stand to gain any personal benefit. How best to accomplish this and in what circumstances is a prudential judgment. For example, I'm sure you have read stories about someone who stops to help someone on the side of the road and ends up getting robbed and murdered. If we are single and without obligations to others we might be more willing to put ourselves in risky situations, whereas when we have a wife and children, we're more cautious of our personal safety on our family's behalf. So, understood in this sense, they're not saying contradictory things, but they're approaching a similar question from two different, complimentary aspects.