OK, so we look at the visitor visa rules initially (these apply to US citizens entering either on a visitor visa, or the visa exemption). Under "permitted activities for all visitors, we find this section:
Business – general activities
5 A visitor may:
- (a) attend meetings, conferences, seminars, interviews;
- (b) give a one-off or short series of talks and speeches provided these are not organised as commercial events and will not make a profit for the organiser;
- (c) negotiate and sign deals and contracts;
- (d) attend trade fairs, for promotional work only, provided the visitor is not directly selling;
- (e) carry out site visits and inspections;
- (f) gather information for their employment overseas;
- (g) be briefed on the requirements of a UK based customer, provided any work for the customer is done outside of the UK.
So, it is understood that business visitors may come to the UK for all these activities (and a whole bunch more are permitted further down the list, provided they are within the same company, or a company that there is an existing business relationship with).
It is clearly understood that, as these visits are "on business" people will be receiving their normal salary for this. The person in question won't be deriving any personal profit from this, and in fact, neither is their employer, who has in fact paid for this privilege. This would seem to me to mean that there isn't any requirement to look at the additional rules for "permitted paid engagements".
However, I can imagine things being complicated somewhat if attending meetings and events was the only thing a person was employed to do. While obviously difficult to enforce, the general understanding for UK immigration is that business visitors will not continue to do their "normal" work while visiting, as that is not listed in the permitted activities section. This obviously gets more complicated if their "normal work" is attending events like this, as it gets harder to argue that permitted paid engagements are not relevant.
Of course, as with all immigration situations, there's a degree of interpretation on the part of the immigration officer that you have to deal with.
Furthermore, I am not an immigration expert, just someone with the rules in front of me (and having seen company reps at various conferences). Note that our resident ex-immigration lawyer appears to disagree with this assessment.