scales = "free" is the secret:
ggplot(spend_opt_tv, aes(x = Daypart, y = Current, fill = Daypart)) +
geom_col(show.legend = FALSE) +
facet_grid(Network ~ ., switch = "y", scales="free") +
coord_flip() +
theme_classic() +
scale_fill_brewer(palette="Dark2") +
theme(axis.text.x = element_text(angle = 90, hjust = 1)) +
theme(axis.title.y = element_blank())

Created on 2019-01-31 by the reprex package (v0.2.1)