WHEN it comes to erecting barriers, Brad Wall lives up to his name. On December 6th the premier of Saskatchewan banned lorries registered in neighbouring Alberta from new government-backed roadworks in his province. When a week later Alberta complained that this violated an interprovincial trade deal, Mr Wall thundered: “We won’t be backing off.” Saskatchewanians faced similar discrimination in Alberta, he said.

In theory Canada’s ten provinces and three territories form a vast single market where goods, services and people move freely. Yet provincial leaders find ways to protect their workers and businesses against rivals next door. Before 2008 Quebec banned yellow margarine, lest its shoppers mistake the grain-based spread (shipped in from the west) for proper butter (churned by its own dairy farmers). In 2012 a man from New Brunswick was arrested and fined for bringing alcohol back home from Quebec, violating a provincial prohibition on booze sold outside state-owned liquor stores; his case has reached Canada’s Supreme Court.

By one estimate, such barriers cost Canada C$100bn ($78bn) a year. Every now and again provincial leaders vow to live up to their country’s neighbourly image. A few months ago they agreed to let plumbers, welders and other tradesmen move province without having to recertify. The bonhomie never lasts.

Mr Wall’s lorry war looks particularly frivolous. In explaining the premier’s decision, his economy minister, Steven Bonk, griped that Alberta discriminates against out-of-province beer (perhaps worried that thirsty Saskatchewan workers there were being deprived of their own lager). The Saskatchewan Heavy Construction Association, a lobby group, grouses about the unfair advantage Albertan drivers enjoy because they pay no provincial sales tax on vehicles.

Alberta denies mistreating anyone. Cody Bexson of the Lloydminster Construction Association, which represents workers in both provinces, has heard no reports of Saskatchewanians being turned away from building sites in Alberta—nor of Albertans in Saskatchewan.

The suspicion is that the spat is less about trade than politics. Ever since Justin Trudeau led his centre-left Liberals to victory in the general election of 2015, Mr Wall has emerged as one of Canada’s loudest conservative voices. He harangues Mr Trudeau’s government, and Alberta’s provincial one, for things like letting in too many refugees and backing a carbon tax. The licence-plate ban could be the last hurrah for Mr Wall, who plans to retire in January. If a dispute panel finds it illegal, Saskatchewan could be fined C$5m. The cost to its reputation may be greater.