Is it appropriate to say "selection of" or "selection for" an allele that evolves by genetic hitchhiking?
The expressions "selection for" and "selection of", their definitions and their usage have been subject to a number of semantic issues throughout the literature. Good and Griffith (1995) is a good read on the subject.

I would argue that neither "selection for" or "selection of" is appropriate to use in the context of genetic hitchhiking because, in this context, by definition, the increase in frequency of an allele is only due to a different locus nearby that is under selection. So an allele that evolves due to hitchhiking is not directly under selection and only changes frequency indirectly due to nearby selection on a linked site.