Okay, with all due respect to people selling compressed air for paintball enthusiasts...
1.) Dive shops generally require a scuba cert. card to sell you compressed air. This card is taken as proof that you have been informed of the risks & methods for dealing with breathing compressed air at depth and changing depth, have demonstrated the knowledge & skill to do so safely, and are making an informed decision to receive and use this potentially dangerous product.
2.) Scuba divers are accustomed to not holding their breath.
If you happen to meet a free diver at depth, and give him gas, you have no way to definitively verify that he is scuba certified. Upon getting a breath, he's likely to break away from you & head for the surface. What do you do? Do you grab him & try to make him ascend gradually with you? A stranger could misinterpret that and try to fight you off. And this guy is trained & conditioned to hold his breath till he reaches the surface.
The only scenario in which such a thing might seem acceptable is if the scuba diver & free diver know each other, the latter is scuba certified & the former knows this, and even then, you've got a free diver who's engaged in a free dive who is used to holding his breath on the way up. Why would this scenario arise? Are we assuming a free diver who wants to free dive deeper than he safely can, and wants to plan to meet his scuba friend at the deep portion of the dive so he can get a fresh breath & do a CESA?
Richard.