It's much worse for them actually. They're obliged to offer this for no more than the cost, unlike a typical "cost plus" basis on which they could profit even minimally - and yet they're also obliged to fulfill all orders from anybody. The offer isn't valid only for whoever you gave binaries to, it's an offer to any third party that's what the requirement says.

In the CD era, it might have made sense to go with written offer if the source is far larger than the binary you ship (e.g you ship a 500MB game on CD, but the source would be 1400MB so that's like 3 CDs, ugh) and you're happy to periodically pay the office intern to burn some source CDs and post them off for the inquisitive customer who asked for them.

In the Internet era it definitely doesn't make sense. Just pop a link to the source next to the binaries and don't sweat it.

Unless, of course, you have no intention of complying anyway.