I think you will find that remain were fined as well
Best for Our Future given a £2,000 penalty for failing to submit documents about donations from Unison (£10,000) and the GMB (£20,000).
Stronger In campaign £1,250 fine.
LibDems fined £18,000 for failing to provide acceptable invoices or receipts for 80 payments with a total value of more than £80,000.
Nothing like the £70,000 penalty for Leave.EU for the lack of transparency of £8,000,000 of funding plus criminal investigations though eh?
Following its investigation into funding for the 2016 EU referendum, the Electoral Commission has referred: Better for the Country, the organisation that ran the Leave.EU referendum campaign; Arron Banks; Leave.EU; Elizabeth Bilney; and other associated companies and individuals. The National Crime Agency has now launched a criminal investigation.
The investigation focused on £2m reported to have been loaned to Better for the Country by Arron Banks and his group of insurance companies and a further £6m reported to have been given to the organisation, on behalf of Leave.EU, by Arron Banks alone.
£2.9m of this money was used to fund referendum spending on behalf of Leave.EU and donations to other campaign groups during the EU referendum.
Following its investigation, the Commission has reasonable grounds to suspect that:
Mr Banks was not the true source of the £8m loans made to Better for the Country.
Loans to Better for the Country, on behalf of Leave.EU, involved a non-qualifying or impermissible company – Rock Holdings Limited, which is incorporated in the Isle of Man.
Arron Banks, Elizabeth Bilney and others involved in Better for the Country, Leave.EU and associated companies concealed the true details of these financial transactions.
A number of criminal offences may have been committed.
Due to multiple suspected offences, some of which fall outside the Commission’s remit, the Commission has referred this matter and handed its evidence to the National Crime Agency.
You can try to spin the "oooh, but Remain campaigns were fined too" but there really is no comparison.