Standard practice in business of all types is to take clients out for a meal to talk business. Usually, the meal setting enables a different type of legitimate, sober interaction. Many types of business are conducted this way. Some companies have policies that limit the value of what a salesperson can share with a client, for example, Applied Materials limits the value of any type of entertainment by a vendor to $100. This is good corporate policy to inhibit undue influence by vendors.
But it is not 'a payment'.
Likewise, it is pretty easy to see that pharma would want a Dr. who is prescribing their medication and has a positive story to tell to speak at one of their seminars. The Dr. might say that his time is worth $x, and the Pharma needs to cover his travel expenses, and then he'd consent to presenting. In this case, any fees paid would be considered payment. The question is, how much is being paid and does that payment present undue influence. Many doctors are independent contractors and can choose to do this type of activity without a policy to override or limit the value of it. On the other hand, state medical boards which license physicians should have policies that limit all medical and pharmaceutical companies in how they can influence physicians.