BRP with PR - passport stamping on arrival at LHR

My wife has a biometric residence permit (BRP) confirming her permanent residence status as family member of an EEA national (i.e. me). She's held this for over a year now, and travels quite a bit with it.

Recently we have noticed that upon arrival into Heathrow, it increasingly happens that (1) a stamp is placed in her passport with the number of her BRP written above it, and (2) she gets digitally fingerprinted.

Neither used to happen in the initial months of her having her BRP - although admittedly at that time, most immigration officers had no idea what an EEA BRP was, which caused lots of minor issues in its own right as they kept arguing she should use the non-UK/EEA channel and/or complete a landing card. Those niggles appear to be largely gone now as the BPR becomes more prevalent (I presumes)...

Last time we travelled back together, I queried the immigration officer about it after she stamped the passport. I told her that it was my firm and clear understanding that under the EEA immigration regulations, UKBF is not supposed / allowed to stamp passport of anyone holding EEA residence.

She was very nice (not always the case) and explained that it was a recent policy decision of Heathrow UKBF to always stamp passports of BRP holders - even if they are EEA nationals. She also said that she was aware that other ports use different rules, e.g. London City airport does not stamp.

I don't know if she was right or merely trying to explain away her own mistake in a friendly way. Does anyone know? And if this is indeed Heathrow UKBF policy, on what grounds do they justify it given that it contravenes the Regulations?

I also queried her about the fingerprinting, and she said that at Heathrow this is now normal for BPR holders. Here I am simply not sure what the rules are - as it appears very random whether or not fingerprints get checked on arrival. Again I would be grateful if anyone could clarify what the rules are on fingerprinting EEA BPR holders...

It is contrary to EU law and UK EEA Regulations to entry stamp passports of EEA family members.

Endorsing the passports of residence card holders
Regulation 11(3)
A Border Force officer may not place a stamp in the passport of a person who holds a residence card when he is admitted to the UK, even if they do not hold an EEA family permit.

Regulation 11(3) expressly prohibits an officer from endorsing the passport of a person who holds a valid residence card or permanent residence card. In addition these passengers are not required to fill in a landing card and should not be asked to do so.

Ref http://www.eearegulations.co.uk/Latest/ByPage/part2_11

Security and fingerprint checks are a domestic security matter.