Offering to pay for lunch is not a "bribe" per se, but it does set up a host-guest dynamic, giving the salesperson a host's rights: the ability to speak first, the right to subtly steer the conversation, the right to offer to extend the meal by suggesting another drink or another desert, the right to invite someone else- a finance company rep, a sales engineer, a satisfied customer- to share the meal. It also gives the customer guest's rights: the right to excuse oneself when the conversation has gone on long enough, the right to refuse alchohol or desert or coffee or other attempts to extend the meal, the right to ask to be convinced further.
Besides, seeing how the salesperson treats the waiter or waitress is instructive; a person who acts like a jerk to the waiter or stiffs them on the tip unprovoked is a person who will rip you off if they ever get the chance. I'm sure there are exceptions to the rule but I've never witnessed it. The point is, seeing how a person acts in an informal setting is invaluable when deciding if you should pursue a business relationship with that person and their company.
Sales isn't just about numbers. Sales is about people. Do I want to get involved with this person and their product? It's not always easy to tell. Luckily, we have a way of getting to know the person behind the product, and part of this process is the business lunch.
The business dinner, however, is usually infinitely sleazier. I've taken to lunch and have been taken to lunch, and usually good things have developed from it, but I've only ever had one good business relationship come out of a dinner, and now avoid them. People going to lunch are going back to the office after. People going to dinner want to have fun after, and then you start veering into bribery territory, with concert tickets or sporting event tickets or activities even less family friendly. A salesperson wanting to take you to dinner, especially when you're attending an out-of-town convention, has a weak product and a too-large marketing budget, generally.