Inductive effect is introduced in many textbooks by giving an example of an alkyl halide $R-X$.
The bond C1-X is polar due to electronegativity difference, therefore X gains partial negative charge ∆(-), and C1 gains partial positive ∆(+). Due to some deficiency of electrons in the C1 , C2-C1 bond becomes polar and above process is repeated.
So by this point of view, if we take $R-Li$, as C1-Li bond is polar, Li gains ∆(+) and C gains ∆(-) and above thinking process is repeated again.
Thus we can conclude that Carbon chain can act as both +I and -I depending upon the situation on hand.
So why do we always consider an alkyl chain to be a +I group?
Example, if on a carbon chain, there is a carbanion, we still consider alkyl group to be +I, but it should have been the opposite.