/usually because they are lying. They think they can pass off the problem to the new buyer. It needs a charge because there is a leak that could be anything from simple to bad compressor seals. I agree with you, if that's "all it needs." let the seller do it. Otherwise, suggest they meet you at a shop and discount the sale price
by the estimated cost to fix it.
'95 9c1, pcmperformance.com 93 tune, Dynomax Ultraflo catback, K & N box filter, LT4 KM, 160 stat, 3.08 Posi.