Do we know if there serious about this? I'd hope the union negotiators would realize there was likely to be a backlash to this and put out this request as a bargaining point they won't fight very hard for.
If they were doing as you say, asking for something they're unlikely to get as a negotiation tactic, they should have asked for commissions. Salesmen getting commissions would be perceived as completely reasonable. It degrade the customer experience by making the salesmen pushier, but commission for salesmen is totally conventional while tips for salesmen is pants-on-head dumb and bizarre.